Prevent a bad .Psize expression from triggering a memory access violation.

	PR 27384
	* listing.c (listing_psize): Check the result of the width
	expression before assigning it to paper_width.
	* testsuite/gas/all/pr27384.s: New test source file.
	* testsuite/gas/all/pr27384.d: New test control file.
	* testsuite/gas/all/pr27384.err: Expected errors from new test.
	* testsuite/gas/all/gas.exp: Run the new test.