Disassembly unit test: memory error

This patch adds a unit test about memory error occurs on reading
memory, and check MEMORY_ERROR exception is always thrown.

gdb:

2017-01-26  Yao Qi  <yao.qi@linaro.org>

	* disasm-selftests.c (memory_error_test): New function.
	(_initialize_disasm_selftests): Register memory_error_test.