ALSA: usb-midi: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertenly introduced[3] to the codebase from now on.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200211194224.GA9383@embeddedorSigned-off-by: NTakashi Iwai <tiwai@suse.de>

sound/usb/midi.c

@@ -91,7 +91,7 @@ struct usb_ms_endpoint_descriptor {
 	__u8  bDescriptorType;
 	__u8  bDescriptorSubtype;
 	__u8  bNumEmbMIDIJack;
-	__u8  baAssocJackID[0];
+	__u8  baAssocJackID[];
 } __attribute__ ((packed));
 
 struct snd_usb_midi_in_endpoint;