Fix segfault processing nios2 pseudo-instructions with too few arguments.

Fix segfault processing nios2 pseudo-instructions with too few arguments.

2017-10-16  Sandra Loosemore  <sandra@codesourcery.com>
	    Henry Wong  <henry@stuffedcow.net>

	gas/
	* config/tc-nios2.c (nios2_translate_pseudo_insn): Check for
	correct number of arguments.
	(md_assemble): Handle failure of nios2_translate_pseudo_insn.
	* testsuite/gas/nios2/illegal_pseudoinst.l: New file.
	* testsuite/gas/nios2/illegal_pseudoinst.s: New file.
	* testsuite/gas/nios2/nios2.exp: Add illegal_pseudoinst test.