Bail in bounds_of_var_in_loop if scev returns NULL.

Bail in bounds_of_var_in_loop if scev returns NULL.

Both initial_condition_in_loop_num and evolution_part_in_loop_num
can return NULL.  This patch exits if either one is NULL.  Presumably
this didn't happen before, because adjust_range_with_scev was called
far less frequently than in ranger, which can call it for every PHI.

gcc/ChangeLog:

	PR tree-optimization/100349
	* vr-values.c (bounds_of_var_in_loop): Bail if scev returns
	  NULL.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr100349.c: New test.