mwifiex: fix double free

clang static analysis reports this problem:

sdio.c:2403:3: warning: Attempt to free released memory
        kfree(card->mpa_rx.buf);
        ^~~~~~~~~~~~~~~~~~~~~~~

When mwifiex_init_sdio() fails in its first call to
mwifiex_alloc_sdio_mpa_buffer, it falls back to calling it
again.  If the second alloc of mpa_tx.buf fails, the error
handler will try to free the old, previously freed mpa_rx.buf.
Reviewing the code, it looks like a second double free would
happen with mwifiex_cleanup_sdio().

So set both pointers to NULL when they are freed.

Fixes: 5e6e3a92 ("wireless: mwifiex: initial commit for Marvell mwifiex driver")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201004131931.29782-1-trix@redhat.com

drivers/net/wireless/marvell/mwifiex/sdio.c

@@ -2403,6 +2403,8 @@ static int mwifiex_alloc_sdio_mpa_buffers(struct mwifiex_adapter *adapter,
 		kfree(card->mpa_rx.buf);
 		card->mpa_tx.buf_size = 0;
 		card->mpa_rx.buf_size = 0;
+		card->mpa_tx.buf = NULL;
+		card->mpa_rx.buf = NULL;
 	}
 
 	return ret;