mosquitto: fix crash on client disconnect

Patch backported from upstream.  Was only noticed on musl and freebsd
clients.  See https://bugs.eclipse.org/bugs/show_bug.cgi?id=475707

Fixes github issue #1692

Signed-off-by: Karl Palsson <karlp@remake.is>

net/mosquitto/Makefile

@@ -10,7 +10,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=mosquitto
 PKG_VERSION:=1.4.3
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 PKG_LICENSE:=BSD-3-Clause
 PKG_LICENSE_FILES:=LICENSE.txt
 

net/mosquitto/patches/001-free-counting-bug-475707.patch

@@ -0,0 +1,14 @@
+diff --git a/lib/memory_mosq.c b/lib/memory_mosq.c
+index b1f5bfd..4d8217d 100644
+--- a/lib/memory_mosq.c
++++ b/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);