list keywords: restore 2.0 similar output

Unset was previously displayed if the application layer was not
set. Code before this patch was displaying '(null)' which is not
user friendly.

src/detect.c

@@ -4902,8 +4902,8 @@ static inline void SigMultilinePrint(int i, char *prefix)
 void SigTableList(const char *keyword)
 {
     size_t size = sizeof(sigmatch_table) / sizeof(SigTableElmt);
-
     size_t i;
+    char *proto_name;
 
     if (keyword == NULL) {
         printf("=====Supported keywords=====\n");
@@ -4928,8 +4928,8 @@ void SigTableList(const char *keyword)
                     printf("%s", sigmatch_table[i].desc);
                 }
                 /* Build feature */
-                printf(";%s;",
-                       AppLayerGetProtoName(sigmatch_table[i].alproto));
+                proto_name = AppLayerGetProtoName(sigmatch_table[i].alproto);
+                printf(";%s;", proto_name ? proto_name : "Unset");
                 PrintFeatureList(sigmatch_table[i].flags, ':');
                 printf(";");
                 if (sigmatch_table[i].url) {