Avoid segfault on invalid directory table

Avoid segfault on invalid directory table

gdb was segfaulting during backtrace on a binary here, where
fe->dir_index parsed from the DWARF info was seen to access beyond the
provided include_dirs array.

This commit bounds the access to entries actually written to the
array, and was verified to output the backtrace correctly.

gdb/ChangeLog:

	* dwarf2read.c (setup_type_unit_groups): Ensure dir_index doesn't
	reference beyond the 'lh->include_dirs' array before accessing to
	it.
	(psymtab_include_file_name): Likewise.
	(dwarf_decode_lines_1): Likewise.
	(dwarf_decode_lines): Likewise.
	(file_file_name): Likewise.