stress-mmaptorture: ensure n is initialized for mappings_unmap exit path

Currentl n is not initialized, and exit path via mappings_unmap uses
this as an for-loop upper bounds check. Ensure it is initialized at
declaration time.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>

Author: ColinIanKing

stress-mmaptorture.c

@@ -459,7 +459,7 @@ static int stress_mmaptorture_child(stress_args_t *args, void *context)
 	do {
 		unsigned char vec[MMAP_SIZE_MAP];
 		uint8_t * CLOBBERED ptr;
-		CLOBBERED size_t n;
+		CLOBBERED size_t n = 0;
 		CLOBBERED size_t mmap_size;
 		CLOBBERED pid_t pid = -1;
 		CLOBBERED uint64_t total_bytes = 0;