Fix msp430 build with gcc-5

Fix msp430 build with gcc-5

gcc-5 correctly complains "loop exit may only be reached after
undefined behavior".  I was going to correct this by checking the
index before dereferencing the array rather than the other way around,
but then I noticed it is possible for extract_cmd to write the
terminating zero one past the end of "cmd".  Fixing that means no
index check is needed in md_assemble.

	* config/tc-msp430.c (md_assemble): Correct size passed to
	extract_cmd.  Remove index check.