ltrimstr/rtrimstr was ignoring and leaking the error returned by
f_startswith()/f_endswith().
This also means that they just let the input pass through for non-string
inputs or arguments.
Only fix the leak for now; in the next release, #2969 will make them
rethrow the error returned by startswith/endswith.
Ref: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=64946