[gdb/symtab] Remove superfluous end-of-sequence marker

While working on PR26935 I noticed that when running test-case
gdb.base/morestack.exp with target board unix/-m32/-fPIE/-pie and ld linker,
I get this linetable fragment for morestack.S using readelf -wL:
...
CU: ../../../../libgcc/config/i386/morestack.S:
Line number    Starting address    View    Stmt
109               0xc9c               x
  ...
838               0xe03               x
  -               0xe04

636                   0               x
637                 0x3               x
  -                 0x4
...
but with "maint info line-table" I get:
...
INDEX  LINE   ADDRESS            IS-STMT
0      END    0x00000004         Y
1      109    0x00000c9c         Y
  ...
110    838    0x00000e03         Y
111    END    0x00000e04         Y
...

So, apparently the entries with addresses 0x0 and 0x3 are filtered out
because the addresses are out of range, but the same doesn't happen with the
end-of-seq terminator.

Fix this by filtering out end-of-seq terminators that do not actually
terminate anything.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-01-04  Tom de Vries  <tdevries@suse.de>

	* buildsym.c (buildsym_compunit::record_line): Filter out end-of-seq
	terminators that do not terminate anything.

gdb/testsuite/ChangeLog:

2021-01-04  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-out-of-range-end-of-seq.exp: New file.

1 files changed, 7 insertions, 2 deletions

diff --git a/gdb/buildsym.c b/gdb/buildsym.c
index 1d8c579..0f7449f 100644
--- a/gdb/buildsym.c
+++ b/gdb/buildsym.c
@@ -707,13 +707,18 @@ buildsym_compunit::record_line (struct subfile *subfile, int line,
      anyway.  */
   if (line == 0)
     {
+      struct linetable_entry *last = nullptr;
       while (subfile->line_vector->nitems > 0)
 	{
-	  e = subfile->line_vector->item + subfile->line_vector->nitems - 1;
-	  if (e->pc != pc)
+	  last = subfile->line_vector->item + subfile->line_vector->nitems - 1;
+	  if (last->pc != pc)
 	    break;
 	  subfile->line_vector->nitems--;
 	}
+
+      /* Ignore an end-of-sequence marker marking an empty sequence.  */
+      if (last == nullptr || last->line == 0)
+	return;
     }
 
   e = subfile->line_vector->item + subfile->line_vector->nitems++;