- Don't relay on bFormatIndex as an internal array index, since this

- Don't relay on bFormatIndex as an internal array index, since this
field is unreliable and can start with any number.  Use an own internal
array index instead.

- If the read buffer is too small, return a propper error to the calling
functions.  Just check the buffer size if we use the read(2) method since
it doesn't affect mmap(2).

Fixes kernel crashes seen with the M$ LifeCam NX-6000 and internal
(laptop) Sonix chipsets.

Tested by jcs@ (Sonix) and myself (NX-6000).

OK deraadt@