gdb/cli: Improve UX when using list with no args

When using "list" with no arguments, GDB will first print the lines
around where the inferior is stopped, then print the next N lines until
reaching the end of file, at which point it warns the user "Line X out
of range, file Y only has X-1 lines.".  This is usually desirable, but
if the user can no longer see the original line, they may have forgotten
the current line or that a list command was used at all, making GDB's
error message look cryptic. It was reported in bugzilla as PR cli/30497.

This commit improves the user experience by changing the behavior of
"list" slightly when a user passes no arguments.  It now prints that the
end of the file has been reached and recommends that the user use the
command "list ." instead.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30497
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>

gdb/source.c

@@ -1484,6 +1484,22 @@ print_source_lines (struct symtab *s, source_lines_range line_range,
 			   line_range.stopline (), flags);
 }
 
+/* See source.h.  */
+
+int
+last_symtab_line (struct symtab *s)
+{
+  const std::vector<off_t> *offsets;
+
+  /* Try to get the offsets for the start of each line.  */
+  if (!g_source_cache.get_line_charpos (s, &offsets))
+    return false;
+  if (offsets == nullptr)
+    return false;
+
+  return offsets->size ();
+}
+
 
 
 /* Print info on range of pc's in a specified line.  */