toplev.c: Check for null argument to fprintf

toplev.c: Check for null argument to fprintf

Ensure that CF does not equal NULL in function output_stack_usage_1
before calling fprintf.  This fixes the following warning/error:

gcc/toplev.c:976:13: error: argument 1 null where non-null expected [-Werror=nonnull]
  976 |     fprintf (cf, "\\n" HOST_WIDE_INT_PRINT_DEC " bytes (%s)",
      |     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  977 |       stack_usage,
      |       ~~~~~~~~~~~~
  978 |       stack_usage_kind_str[stack_usage_kind]);

An example call side where CF is NULL is in function output_stack_usage.

gcc/ChangeLog:

2020-04-30  Stefan Schulze Frielinghaus  <stefansf@linux.ibm.com>

	* toplev.c (output_stack_usage_1): Ensure that first
	argument to fprintf is not null.