combine.c (if_then_else_cond): Check for NULL return value of simplify_gen_subreg.

ChangeLog:

	* combine.c (if_then_else_cond): Check for NULL return value of
	simplify_gen_subreg.

testsuite/ChangeLog:

	* gcc.dg/20040310-1.c: New test.

From-SVN: r79347