Return -1 on memory error in print_insn_m68k

Return -1 on memory error in print_insn_m68k

m68k-dis.c:print_insn_m68k doesn't return -1 on memory error, but GDB
expects it returning -1 on memory error.

opcodes:

2017-01-13  Yao Qi  <yao.qi@linaro.org>

	* m68k-dis.c (match_insn_m68k): Extend comments.  Return -1
	if FETCH_DATA returns 0.
	(m68k_scan_mask): Likewise.
	(print_insn_m68k): Update code to handle -1 return value.