audio/festival: fix crash due to overly aggressive optimization

Modern clang, optimize away checks like "if (this == NULL)", which
are, indeed, redundant in good C++ code. Unfortunately, festival's
code is not too good, and the checks are essential...

PR: 269583

While here, eliminate some of the other warnings raised by both
compiler and valgrind, and adapt one more patch from Debian.

Bump PORTREVISION.