staging: vchiq: fix uninitialized variable copy

Smatch found a local variable that can get copied to another
local variable without an initializion in the error case:

 drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c:1056 vchiq_get_user_ptr() error: uninitialized symbol 'ptr'.

This seems harmless, as the function should normally get inlined, with
the output directly written or not. In any case, the uninitialized data
is never used after get_user() fails.

As Dan mentions, it could still trigger an UBSAN runtime error, and it
is of course a bad idea to copy uninitialized variables, so just
bail out early.
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210105135256.1810337-1-arnd@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c

@@ -1052,14 +1052,21 @@ static inline int vchiq_get_user_ptr(void __user **buf, void __user *ubuf, int i
 		compat_uptr_t ptr32;
 		compat_uptr_t __user *uptr = ubuf;
 		ret = get_user(ptr32, uptr + index);
+		if (ret)
+			return ret;
+
 		*buf = compat_ptr(ptr32);
 	} else {
 		uintptr_t ptr, __user *uptr = ubuf;
 		ret = get_user(ptr, uptr + index);
+
+		if (ret)
+			return ret;
+
 		*buf = (void __user *)ptr;
 	}
 
-	return ret;
+	return 0;
 }
 
 struct vchiq_completion_data32 {