Make Pod::Html more robust against malformed L<> contents.

Make Pod::Html more robust against malformed L<> contents.

Pod::Html attempts to search for the contents to see if they are a suffix
of any entry in an existing list, using a regular expression.
Previously the contents were interpolated directly into a regex, which
meant that if they happened to be syntactically invalid regular expression
syntax, Pod::Html aborted with a runtime error.