lwip_init(): at least IAR warns about "LWIP_UNUSED_ARG(a)" accessing an uninitialized variable... Try to fix this by initializing it (it will be optimized away anyway)

(cherry picked from commit 946b2315163de66952d48b308551b94b2b1043b4)

src/core/init.c

@@ -337,7 +337,7 @@ void
 lwip_init(void)
 {
 #ifndef LWIP_SKIP_CONST_CHECK
-  int a;
+  int a = 0;
   LWIP_UNUSED_ARG(a);
   LWIP_ASSERT("LWIP_CONST_CAST not implemented correctly. Check your lwIP port.", LWIP_CONST_CAST(void*, &a) == &a);
 #endif