[475707] Fix free related crash on openwrt.

Thanks to Karl Palsson.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=475707

ChangeLog.txt

@@ -5,6 +5,7 @@ Broker:
 - Don't leak sockets when outgoing bridge with multiple addresses cannot
   connect. Closes #477571.
 - Fix cross compiling of websockets. Closes #475807.
+- Fix memory free related crashes on openwrt. Closes #475707.
 
 
 1.4.3 - 20150818

lib/memory_mosq.c

@@ -54,6 +54,9 @@ void *_mosquitto_calloc(size_t nmemb, size_t size)
 void _mosquitto_free(void *mem)
 {
 #ifdef REAL_WITH_MEMORY_TRACKING
+	if(!mem){
+		return;
+	}
 	memcount -= malloc_usable_size(mem);
 #endif
 	free(mem);