stm32f7 example starts on real h/w, with plus-TCP driver

src/str.c

@@ -44,7 +44,7 @@ int mg_vcasecmp(const struct mg_str *str1, const char *str2) {
 struct mg_str mg_strdup(const struct mg_str s) {
   struct mg_str r = {NULL, 0};
   if (s.len > 0 && s.ptr != NULL) {
-    char *sc = (char *) malloc(s.len + 1);
+    char *sc = (char *) calloc(1, s.len + 1);
     if (sc != NULL) {
       memcpy(sc, s.ptr, s.len);
       sc[s.len] = '\0';