eina: silence gcc warning.

Code is correct and won't lead to a case where size is > 0 and buffer
isn't set, but gcc can't see the relation between both variable.

src/lib/eina/eina_debug.c

@@ -535,7 +535,7 @@ _monitor(void *_data)
    // impact the application specifically
    for (;session;)
      {
-        unsigned char *buffer;
+        unsigned char *buffer = NULL;
         int size;
 
         size = _packet_receive(session, &buffer);