ptx: avoid infloop due to zero-length matches with -S regex

ptx: avoid infloop due to zero-length matches with -S regex

* src/ptx.c (find_occurs_in_text): Die with an appropriate error
diagnostic when the given regular expression returns a match of
length 0.
* tests/misc/ptx.pl (S-infloop): Add a test.
* NEWS (Bug fixes): Mention the fix.

Fixes https://bugs.gnu.org/28417 which was detected using
Symbolic Execution techniques developed in the course of the
SYMBIOSYS research project at COMSYS, RWTH Aachen University.