Bug 702582, CVE 2020-15900 Memory Corruption in Ghostscript 9.52

Fix the 'rsearch' calculation for the 'post' size to give the correct
size.  Previous calculation would result in a size that was too large,
and could underflow to max uint32_t. Also fix 'rsearch' to return the
correct 'pre' string with empty string match.

A future change may 'undefine' this undocumented, non-standard operator
during initialization as we do with the many other non-standard internal
PostScript operators and procedures.