pp_ctl.c:pp_require: Remove redundant VMS null checks

SvPVX(newSVpv("",...)) should never return null.
Use NOT_REACHED in one more place in op.c

Under clang, this reduces the size of op.o by 16 bytes.

$ clang -v
Apple LLVM version 4.2 (clang-425.0.27) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.5.0
Thread model: posix