Fix undefined behavior

Without the casts the bytes accesses get converted to int. but int is
not guaranteed to be 4 bytes large. Even when it is 4 bytes large
`bytes[3] << 24` does not fit because int is signed.

Signed-off-by: Valentin Kettner <vakevk@gmail.com>

cursor/xcursor.c

@@ -285,10 +285,11 @@ _XcursorReadUInt (XcursorFile *file, XcursorUInt *u)
 
     if ((*file->read) (file, bytes, 4) != 4)
 	return XcursorFalse;
-    *u = ((bytes[0] << 0) |
-	  (bytes[1] << 8) |
-	  (bytes[2] << 16) |
-	  (bytes[3] << 24));
+
+    *u = ((XcursorUInt)(bytes[0]) << 0) |
+         ((XcursorUInt)(bytes[1]) << 8) |
+         ((XcursorUInt)(bytes[2]) << 16) |
+         ((XcursorUInt)(bytes[3]) << 24);
     return XcursorTrue;
 }