xtensa: fix gas segfault with --text-section-literals

xtensa: fix gas segfault with --text-section-literals

When --text-section-literals is used and code in the .init or .fini
emits literal in the absence of .literal_position, xtensa_move_literals
segfaults.

Check that search_frag is non-NULL in the xtensa_move_literals and
report error otherwise.

2015-05-26  Max Filippov  <jcmvbkbc@gmail.com>
gas/
	* config/tc-xtensa.c (xtensa_move_literals): Check that
	search_frag is non-NULL. Report error if literal frag is not
	found.