Throw in "& 0xf" to quiet Coverity's anxieties.

Throw in "& 0xf" to quiet Coverity's anxieties.

The compiler on my Mac (a Clang release) appears to know enough not to
waste the CPU's time ANDing a value between 0 and 15 with 0xf, even
though the lack of that ANDing appears to give Coverity sleepless
nights.

This should squelch a pile of CIDs.  It also stuffs the
octet-to-two-hex-digits code into an inline routine.