spvbin-helpers: Properly handle parsing strings with no destination.

spvbin-helpers: Properly handle parsing strings with no destination.

binary-parser-generator will pass a null pointer to spvbin_parse_string()
or spvbin_parse_bestring() for a string that should be parsed but not
stored.  In such a case, the implementation would dereference a null
pointer.  This fixes the problem.

This is only a theoretical problem because the grammars PSPP uses now
always store the strings that they parse.

Found by cppcheck.
Reported by John Darrington.