Courtesy of IRIX cc: fix checking of Fwrite() return codes.

"pack.c", line 509: warning(1183): pointless comparison of unsigned integer
          with zero
  	if (Fwrite(buf, sizeof(buf[0]), count, fd) < 0) {

This is particularly nasty since the compiler *optimizes out* the call to
Fwrite(), producing .rpms which aren't really all there.

Patches coming up...