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.

src/log-pcap.c

@@ -1418,10 +1418,8 @@ static OutputInitResult PcapLogInitCtx(ConfNode *conf)
                 compression_str ? compression_str : "none");
     }
 
-    if (filename) {
-        if (ParseFilename(pl, filename) != 0)
-            exit(EXIT_FAILURE);
-    }
+    if (ParseFilename(pl, filename) != 0)
+        exit(EXIT_FAILURE);
 
     SCLogInfo("using %s logging", pl->mode == LOGMODE_SGUIL ?
               "Sguil compatible" : (pl->mode == LOGMODE_MULTI ? "multi" : "normal"));