tc-pdp11.c: remove useless code

tc-pdp11.c: remove useless code

if the condition is true then we know that str already points to a'\0' in the
string passed to the function.  Since we know the latter part of the function
doesn't modify that string, and str already points to a null byte there's no
point in changing str to point to a literal empty string.

gas/ChangeLog:

2016-03-22  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-pdp11.c (md_assemble): Remove useless if and assignment to
	str.