[psaux] Fix another assertion.

* src/psaux/psintrp.c (cf2_interpT2CharString)
<cf2_escCALLOTHERSUBR>: Convert assertion into error, since the
problem can happen with invalid user input.

Test case is file

  fuzzing/corpora/legacy/oss-fuzz/5754332360212480-unknown-read

in the `freetype2-testing` repository.
2 files changed