[PATCH] Fix an out of bounds array access in, find_epilogue_using_linetable

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

This causes random test failures like these:

FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: $fba_value == $fn_fba
FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: check frame-id matches
FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: bt 2
FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: up
FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: $sp_value == $::main_sp
FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: $fba_value == $::main_fba
FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: [string equal $fid $::main_fid]

Here the read happens below the first element of the line
table, and the test failure depends on the value that is
read from there.

Theoretically it is also possible that std::lower_bound
returns a pointer exactly at the upper bound of the line table,
also here the read value is undefined, that happens in this test:

FAIL: gdb.dwarf2/dw2-epilogue-begin.exp: confirm watchpoint doesn't trigger

Fixes: 528b729be1a2 ("gdb/dwarf2: Add support for DW_LNS_set_epilogue_begin in line-table")
---
 gdb/symtab.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/gdb/symtab.c b/gdb/symtab.c
index 86603dfebc3..2fc8e819932 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -4177,11 +4177,10 @@ find_epilogue_using_linetable (CORE_ADDR func_addr)
 	   return lte.unrelocated_pc () < pc;
 	 });
 
-      while (it->unrelocated_pc () >= unrel_start)
+      while (it > linetable->item && (--it)->unrelocated_pc () >= unrel_start)
       {
 	if (it->epilogue_begin)
 	  return {it->pc (objfile)};
-	it --;
       }
     }
   return {};
-- 
2.39.2