When awk reaches EOF parsing the program file, curpfile is incremented.
However, cursource() uses curpfile without checking it against npfile
which can cause an out of bounds access of pfile[] if there is a syntax
error at the end of the program file.
* Cast to uschar when storing a char in an int that will be used as an index.
Fixes a heap underflow when the input char has the high bit set and
FS is a regex.
* Add regress test for underflow when RS is a regex and input is 8-bit.
