detect-mqtt: unify error handling in rule parsing

This is meant to provide a single path to the error case.
This might help make things more clear for static
checkers.

src/detect-mqtt-connack-sessionpresent.c

@@ -154,8 +154,8 @@ static bool *DetectMQTTConnackSessionPresentParse(const char *rawstr)
     return de;
 
 error:
-    if (de != NULL)
-        SCFree(de);
+    /* de can't be NULL here */
+    SCFree(de);
     return NULL;
 }
 

src/detect-mqtt-connect-flags.c

@@ -209,8 +209,8 @@ static DetectMQTTConnectFlagsData *DetectMQTTConnectFlagsParse(const char *rawst
     return de;
 
 error:
-    if (de != NULL)
-        SCFree(de);
+    /* de can't be NULL here */
+    SCFree(de);
     return NULL;
 }
 

src/detect-mqtt-flags.c

@@ -189,8 +189,8 @@ static DetectMQTTFlagsData *DetectMQTTFlagsParse(const char *rawstr)
     return de;
 
 error:
-    if (de != NULL)
-        SCFree(de);
+    /* de can't be NULL here */
+    SCFree(de);
     return NULL;
 }