host:  CID fixes for ezusb.c - Unchecked return values and user-provided values

Addressing items picked up by the Coverity scan:
    CID 1042551, 1042552, 1042546, 1042537

These items consisted of unchecked return values, and using some
length/size values provided in file headers directly.

Added return value checks and some quick sanity checks on length values.

Did some misc. cleanup on debug output.