[gdb/symtab] Fix segfault in process_psymtab_comp_unit

When running test-case gdb.dwarf2/dw2-dummy-cu.exp without -readnow, we run
into:
...
(gdb) file outputs/gdb.dwarf2/dw2-dummy-cu/dw2-dummy-cu^M
Reading symbols from outputs/gdb.dwarf2/dw2-dummy-cu/dw2-dummy-cu...^M
ERROR: Couldn't load dw2-dummy-cu into GDB (eof).
...

The problem is that we're running into a segfault:
...
Thread 1 "gdb" received signal SIGSEGV, Segmentation fault.
process_psymtab_comp_unit (this_cu=0x2141090, per_objfile=0x1aa4140,
    want_partial_unit=false, pretend_language=language_minimal)
    at /home/vries/gdb_versions/devel/src/gdb/dwarf2/read.c:7023
7023      switch (reader.comp_unit_die->tag)
...
due to reader.comp_unit_die == nullptr:
...
(gdb) p reader.comp_unit_die
$1 = (die_info *) 0x0
...

Indeed, there's no CU DIE in the test-case:
...
$ readelf -wi outputs/gdb.dwarf2/dw2-dummy-cu/dw2-dummy-cu
Contents of the .debug_info section:

  Compilation Unit @ offset 0x0:
   Length:        0x7 (32-bit)
   Version:       2
   Abbrev Offset: 0x0
   Pointer Size:  4
$
...

Fix this by handling reader.comp_unit_die == nullptr in
process_psymtab_comp_unit.

Update the test-case to trigger this PR, as per PR27920 - "[gdb/testsuite]
hardcoding -readnow skips testing of partial symbols".

Tested on x86_64-linux.

gdb/ChangeLog:

2021-05-27  Tom de Vries  <tdevries@suse.de>

	PR symtab/27919
	* dwarf2/read.c (process_psymtab_comp_unit):

gdb/testsuite/ChangeLog:

2021-05-27  Tom de Vries  <tdevries@suse.de>

	PR symtab/27919
	PR testsuite/27920
	* gdb.dwarf2/dw2-dummy-cu.exp: Use maint expand-symtabs instead of
	-readnow.

4 files changed, 16 insertions, 5 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 3c0c2db..6170d2d 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2021-05-27  Tom de Vries  <tdevries@suse.de>
 
+	PR symtab/27919
+	* dwarf2/read.c (process_psymtab_comp_unit):
+
+2021-05-27  Tom de Vries  <tdevries@suse.de>
+
 	* dwarf2/read.c (find_partial_die): Fix "Cannot not" typo in dwarf
 	error.
 
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 0da87bb..4cc5b4f 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -7020,6 +7020,9 @@ process_psymtab_comp_unit (dwarf2_per_cu_data *this_cu,
 
   cutu_reader reader (this_cu, per_objfile, nullptr, nullptr, false);
 
+  if (reader.comp_unit_die == nullptr)
+    return;
+
   switch (reader.comp_unit_die->tag)
     {
     case DW_TAG_compile_unit:
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 968e623..22a99c0 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,12 @@
 2021-05-27  Tom de Vries  <tdevries@suse.de>
 
+	PR symtab/27919
+	PR testsuite/27920
+	* gdb.dwarf2/dw2-dummy-cu.exp: Use maint expand-symtabs instead of
+	-readnow.
+
+2021-05-27  Tom de Vries  <tdevries@suse.de>
+
 	PR testsuite/27921
 	* gdb.dwarf2/gdb-index.exp (add_gdb_index): Rename to ...
 	(local_add_gdb_index): ... this.
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp
index 6739724..7871851 100644
--- a/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp
+++ b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp
@@ -27,13 +27,9 @@ if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" object {}] != "
     return -1
 }
 
-set saved_gdbflags $GDBFLAGS
-set GDBFLAGS "$GDBFLAGS -readnow"
-
 clean_restart $executable
+gdb_test_no_output "maint expand-symtab"
 
 # Something simple to verify gdb didn't crash, and has read in whatever symbol
 # info is there.
 gdb_test "info fun main" "main_end"
-
-set GDBFLAGS $saved_gdbflags