tests/tcg/multiarch/linux-test: Fix error check for shmat

The error indicator for this syscall is -1, not 0.

Fixes: e374bfa ("shm tests - disabled clone test")
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191106113318.10226-3-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

tests/tcg/multiarch/linux-test.c

@@ -503,8 +503,9 @@ static void test_shm(void)
 
     shmid = chk_error(shmget(IPC_PRIVATE, SHM_SIZE, IPC_CREAT | 0777));
     ptr = shmat(shmid, NULL, 0);
-    if (!ptr)
+    if (ptr == (void *)-1) {
         error("shmat");
+    }
 
     memset(ptr, 0, SHM_SIZE);