{src,util}: Use NULL instead of 0 for pointer

Change-Id: I75fa4577055f25dde0a8b1044c005bba72cabd92
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/26857
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/libgcov.c

@@ -409,7 +409,7 @@ gcov_exit(void)
 		gcov_position_t summary_pos = 0;
 		gcov_position_t eof_pos = 0;
 		const char *fname, *s;
-		struct gcov_fn_buffer *fn_buffer = 0;
+		struct gcov_fn_buffer *fn_buffer = NULL;
 		struct gcov_fn_buffer **fn_tail = &fn_buffer;
 
 		fname = gi_ptr->filename;