gdb: Handle requests to print source lines backward

...by which I mean from high line number to low, not, actually
backward character by character!

Commit:

  commit 62f29fda90cf1d5a1899f57ef78452471c707fd6
  Date:   Tue Oct 9 22:21:05 2018 -0600

      Highlight source code using GNU Source Highlight

introduced a regression in the test gdb.linespec/explicit.exp, in
which a request is made to GDB to print a reverse sequence of lines,
from +10 to -10 from the current line number.  The expected behaviour
is that GDB prints nothing.  The above commit changed this so that GDB
now prints:

  Line number 32 out of range; /path/to/gdb/testsuite/gdb.linespec/explicit.c has 71 lines.

which is a little confusing.

This commit fixes the regression, and restores the behaviour that GDB
prints nothing.

While I was passing I noticed a call to `back` on a std::string that I
was concerned could be empty if the request for source lines returns
an empty string.  I don't know if it would be possible for a request
for lines to return an empty string, I guess it should be impossible,
in which case, maybe this should be an assertion, but adding a `empty`
check, seems like an easy and cheap safety net.

gdb/ChangeLog:

	* source.c (print_source_lines_base): Handle requests to print
	reverse line number sequences, and guard against empty lines
	string.

2 files changed, 17 insertions, 1 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 3ea7576..2ee19f3 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 2019-01-08  Andrew Burgess  <andrew.burgess@embecosm.com>
 
+	* source.c (print_source_lines_base): Handle requests to print
+	reverse line number sequences, and guard against empty lines
+	string.
+
+2019-01-08  Andrew Burgess  <andrew.burgess@embecosm.com>
+
 	* source.c (print_source_lines_base): Fix skip of '\r' if next
 	character is '\n'.
 
diff --git a/gdb/source.c b/gdb/source.c
index e77789c..71da396 100644
--- a/gdb/source.c
+++ b/gdb/source.c
@@ -1346,6 +1346,16 @@ print_source_lines_base (struct symtab *s, int line, int stopline,
 
   last_source_error = 0;
 
+  /* If the user requested a sequence of lines that seems to go backward
+     (from high to low line numbers) then we don't print anything.
+     The use of '- 1' here instead of '<=' is currently critical, we rely
+     on the undefined wrap around behaviour of 'int' for stopline.  When
+     the use has done: 'set listsize unlimited' then stopline can overflow
+     and appear as MIN_INT.  This is a long-standing bug that needs
+     fixing.  */
+  if (stopline - 1 < line)
+    return;
+
   std::string lines;
   if (!g_source_cache.get_source_lines (s, line, stopline - 1, &lines))
     error (_("Line number %d out of range; %s has %d lines."),
@@ -1392,7 +1402,7 @@ print_source_lines_base (struct symtab *s, int line, int stopline,
       if (c == '\0')
 	break;
     }
-  if (lines.back () != '\n')
+  if (!lines.empty() && lines.back () != '\n')
     uiout->text ("\n");
 }