log-pcap: remove redundant check

log-pcap: remove redundant check

Check is always true but confuses cppcheck:

src/log-pcap.c:1224:32: warning: Either the condition 'filename' is redundant or there is possible null pointer dereference: filename. [nullPointerRedundantCheck]
    if ((pl->prefix = SCStrdup(filename)) == NULL) {
                               ^
src/log-pcap.c:1421:9: note: Assuming that condition 'filename' is not redundant
    if (filename) {
        ^
src/log-pcap.c:1224:32: note: Null pointer dereference
    if ((pl->prefix = SCStrdup(filename)) == NULL) {
                               ^

Bug: #5291.
(cherry picked from commit 3dfbf0bf1124aab7c7060f46b364877ab25455ee)