[gdb/symtab] Handle setting line bp without debug line info

When setting a breakpoint on a line in an executable without debug line info,
we run into an abort.

The problem occurs when calling set_default_source_symtab_and_line, which
calls select_source_symtab (0), which is where we try to find the line number
for main:
...
  /* Make the default place to list be the function `main'
     if one exists.  */
  block_symbol bsym = lookup_symbol (main_name (), 0, VAR_DOMAIN, 0);
  if (bsym.symbol != nullptr && SYMBOL_CLASS (bsym.symbol) == LOC_BLOCK)
    {
      symtab_and_line sal = find_function_start_sal (bsym.symbol, true);
      loc->set (sal.symtab, std::max (sal.line - (lines_to_list - 1), 1));
      return;
    }
...

However, due to the missing debug line info, find_function_start_sal returns a
sal with sal.symtab == 0:
...
(gdb) p /x sal
$2 = {pspace = 0x1a4a7f0, symtab = 0x0, symbol = 0x1d9e480, section = 0x1d5b398,
  msymbol = 0x0, line = 0x0, pc = 0x4004ab, end = 0x0, explicit_pc = 0x0,
  explicit_line = 0x0, is_stmt = 0x0, prob = 0x0, objfile = 0x0}
...
which eventually causes an segfault in create_sals_line_offset because
self->default_symtab->filename is accessed while self->default_symtab == NULL.

Fix this by handling sal.symtab == NULL in select_source_symtab.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-10-17  Tom de Vries  <tdevries@suse.de>

	PR symtab/26317
	* source.c (select_source_symtab): Handling sal.symtab == NULL for
	symbol main.

gdb/testsuite/ChangeLog:

2020-10-17  Tom de Vries  <tdevries@suse.de>

	PR symtab/26317
	* gdb.dwarf2/dw2-main-no-line-number.exp: New file.

2 files changed, 73 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 50d21cb..d513797 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2020-10-17  Tom de Vries  <tdevries@suse.de>
 
+	PR symtab/26317
+	* gdb.dwarf2/dw2-main-no-line-number.exp: New file.
+
+2020-10-17  Tom de Vries  <tdevries@suse.de>
+
 	* lib/gdb.exp (standard_testfile): Also treat args starting with '-'
 	as suffix.
 	* gdb.dwarf2/atomic.c: Rename to ...
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-main-no-line-number.exp b/gdb/testsuite/gdb.dwarf2/dw2-main-no-line-number.exp
new file mode 100644
index 0000000..1f62d0c
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-main-no-line-number.exp
@@ -0,0 +1,68 @@
+# Copyright 2020 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Setup a main without line number info, and verify that attempting to
+# set a breakpoint on a line number doesn't crash gdb.
+
+load_lib dwarf.exp
+
+set testname [file rootname [file tail [info script]]]
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    verbose "Skipping $testname test."
+    return 0
+}
+
+# The .c files use __attribute__.
+if [get_compiler_info] {
+    return -1
+}
+if !$gcc_compiled {
+    verbose "Skipping $testname test."
+    return 0
+}
+
+standard_testfile main.c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels Llines
+    global srcdir subdir srcfile
+
+    cu {} {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name $srcfile}
+	} {
+	    subprogram {
+		{external 1 flag}
+		{MACRO_AT_func {main}}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+set breakpoint_at_missing_lineno_set [gdb_breakpoint "1" no-message]
+gdb_assert { !$breakpoint_at_missing_lineno_set  }