Distorted 'ap' voice output when compiling with gcc 8.3

[undernorthernsky]

Building with gcc 8.3.0 (Debian stable, x86_64) the 'ap' voice is broken (the others are fine); similar to #210, but even when generating wav directly:

./mimic -voice ap -t 'hello world' -o broken.wav

broken.wav.gz

Tested using 1.3.0 and current head.
When switching to a different compiler the problem is gone.

./autogen.sh
export CC=gcc-4.9
./configure && make

Clang 7 is also ok, so maybe this is a gcc bug.