Coverity: Forward NULL: CID 439

Coverity: Forward NULL: CID 439

	* src/fns.c (concat): Manually skip and abort when
	string_result_ptr is NULL. Coverity is actually right
	here, because when we are in a fatal error handling,
	assert will be a no-op, so we better just put an
	abort for the other cases and not try to dereference...

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>