Initialize res in get_next_pcs_read_memory_unsigned_integer

This patch initialize res to zero, otherwise, it may have some garbage
bits after the *the_target->read_memory call.

gdb/gdbserver:

2016-05-05  Yao Qi  <yao.qi@linaro.org>

	* linux-arm-low.c (get_next_pcs_read_memory_unsigned_integer):
	Initialize res to zero.