[ARM] Fix NULL dereference of march_ext_opt

Commit 4d354d8b introduced a NULL
pointer dereference by replacing a pointer assignment by a pointer
dereference assignment without adding a NULL pointer check. This patch
fixes it.

2018-03-02  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gas/
	* config/tc-arm.c (md_begin): Add NULL pointer check before
	dereferencing march_ext_opt.