usb: gadget: ncm: Fix handling of zero block length packets

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: ncm: Fix handling of zero block length packets

While connecting to a Linux host with CDC_NCM_NTB_DEF_SIZE_TX

set to 65536, it has been observed that we receive short packets,

which come at interval of 5-10 seconds sometimes and have block

length zero but still contain 1-2 valid datagrams present.

According to the NCM spec:

"If wBlockLength = 0x0000, the block is terminated by a

short packet. In this case, the USB transfer must still

be shorter than dwNtbInMaxSize or dwNtbOutMaxSize. If

exactly dwNtbInMaxSize or dwNtbOutMaxSize bytes are sent,

and the size is a multiple of wMaxPacketSize for the

given pipe, then no ZLP shall be sent.

wBlockLength= 0x0000 must be used with extreme care, because

of the possibility that the host and device may get out of

sync, and because of test issues.

wBlockLength = 0x0000 allows the sender to reduce latency by

starting to send a very large NTB, and then shortening it when

the sender discovers that there’s not sufficient data to justify

sending a large NTB"

However, there is a potential issue with the current implementation,

as it checks for the occurrence of multiple NTBs in a single

giveback by verifying if the leftover bytes to be processed is zero

or not. If the block length reads zero, we would process the same

NTB infintely because the leftover bytes is never zero and it leads

to a crash. Fix this by bailing out if block length reads zero.