checksrc: fix possible endless loops/errors in the banned function logic

checksrc: fix possible endless loops/errors in the banned function logic

By quoting the search expression to be replaced. This avoid the issue
when the code leading up to a banned function contained regex characters
that the script did not explicitly handle, e.g. `+`.

Assisted-by: Daniel Stenberg
Ref: https://perldoc.perl.org/functions/quotemeta
Follow-up to dd37d6970cfd8b4cf47ebd469f03772813b92c23 #18775

Closes #18845