detect/pcre: assist code analyzer around pointer logic

detect/pcre: assist code analyzer around pointer logic

cppcheck:

src/detect-pcre.c:381:27: warning: Either the condition 'pcap' is redundant or there is overflow in pointer subtraction. [nullPointerArithmeticRedundantCheck]
            cut_capture = MIN((pcap - regexstr), (fcap - regexstr));
                          ^
src/detect-pcre.c:378:18: note: Assuming that condition 'pcap' is not redundant
        else if (pcap && !fcap)
                 ^
src/detect-pcre.c:381:27: note: Null pointer subtraction
            cut_capture = MIN((pcap - regexstr), (fcap - regexstr));
                          ^

Bug: #5291.