pcrs_compile_replacement(): Fix a clang warning when building with FUZZ support

The warning was:
"Address of stack memory associated with local variable 'length' is
still referred to by the static variable 'length_stack' upon returning
to the caller. This will be a dangling reference"

For the fuzzing use case it didn't matter but it still shouldn't be done.

While at it, remove a couple of variables that were only used
to make debugging more convenient.