Fix an out of bounds array access in find_epilogue_using_linetable

An out of bounds array access in find_epilogue_using_linetable 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.

It also happens 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")

Co-Authored-By: Tom de Vries <tdevries@suse.de>

PR symtab/31268
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31268