gdb: xtensa: fix frame initialization when PC is invalid

gdb: xtensa: fix frame initialization when PC is invalid

When gdb is used on core dump and PC is not pointing to a readable
memory read_memory_integer call in the xtensa_frame_cache throws an
error, making register inspection/backtracing impossible in that thread.

Use safe_read_memory_integer instead.

2016-03-29  Max Filippov  <jcmvbkbc@gmail.com>
gdb/
	* xtensa-tdep.c (xtensa_frame_cache): Change op1 type to LONGEST.
	Use safe_read_memory_integer instead of read_memory_integer.