HID: prodikeys: Check presence of pm->input_ep82

Fake USB devices can send their own report descriptors for which the
input_mapping() hook does not get called.  In this case, pm->input_ep82 stays
NULL, which leads to a crash later.

This does not happen with the real device, but can be provoked by imposing as
one.

Cc: stable@vger.kernel.org
Signed-off-by: Günther Noack <gnoack@google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

Author: gnoack

drivers/hid/hid-prodikeys.c

@@ -378,6 +378,10 @@ static int pcmidi_handle_report4(struct pcmidi_snd *pm, u8 *data)
 	bit_mask = (bit_mask << 8) | data[2];
 	bit_mask = (bit_mask << 8) | data[3];
 
+	/* robustness in case input_mapping hook does not get called */
+	if (!pm->input_ep82)
+		return 0;
+
 	/* break keys */
 	for (bit_index = 0; bit_index < 24; bit_index++) {
 		if (!((0x01 << bit_index) & bit_mask)) {