filter, stream: Fix error check for DMA-BUF buffers

When a buffer is of type SPA_DATA_DmaBuf, the 'data' field
will be NULL - we instead have to check if the file descriptor
is valid. The current code, however, always checks if the
data is NULL, which gives a false positive for DMA-BUF buffers,
signaling an error when there is none.

Fix that by also checking if the buffer type is MemPtr together
with the NULL check of the 'data' field.