Return -1 on memory error in print_insn_msp430

Return -1 on memory error in print_insn_msp430

Disassemblers in opcodes return -1 on memory error, but msp430 doesn't
follow this convention.  If I change GDB not to throw exception in
disassemble_info.memory_error_func and rely on the return value of
disassembler, I'll get the following output.

(gdb) disassemble 0x0,+8
Dump of assembler code from 0x0 to 0x8:
   0x00000000:  .word   0xffff; ????
   0x00000002:  .word   0xffff; ????
   0x00000004:  .word   0xffff; ????
   0x00000006:  .word   0xffff; ????
End of assembler dump.

This patch teaches print_insn_msp430 and its callees to return -1
on memory error.

opcodes:

2017-01-12  Yao Qi  <yao.qi@linaro.org>

	* msp430-dis.c (msp430_singleoperand): Return -1 if
	msp430dis_opcode_signed returns false.
	(msp430_doubleoperand): Likewise.
	(msp430_branchinstr): Return -1 if
	msp430dis_opcode_unsigned returns false.
	(msp430x_calla_instr): Likewise.
	(print_insn_msp430): Likewise.