of: overlay: validate offset from property fixups

The smatch static checker marks the data in offset as untrusted,
leading it to warn:

  drivers/of/resolver.c:125 update_usages_of_a_phandle_reference()
  error: buffer underflow 'prop->value' 's32min-s32max'

Add check to verify that offset is within the property data.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NFrank Rowand <frank.rowand@sony.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: NRob Herring <robh@kernel.org>

drivers/of/resolver.c

@@ -122,6 +122,11 @@ static int update_usages_of_a_phandle_reference(struct device_node *overlay,
 			goto err_fail;
 		}
 
+		if (offset < 0 || offset + sizeof(__be32) > prop->length) {
+			err = -EINVAL;
+			goto err_fail;
+		}
+
 		*(__be32 *)(prop->value + offset) = cpu_to_be32(phandle);
 	}