Add mg_strfree()

CL: Add mg_strfree()

PUBLISHED_FROM=b80566218986e0607a9d2758bf776a1031b27212

mongoose.h

@@ -2280,6 +2280,11 @@ int mg_strcmp(const struct mg_str str1, const struct mg_str str2);
  */
 int mg_strncmp(const struct mg_str str1, const struct mg_str str2, size_t n);
 
+/*
+ * Free the string (assuming it was heap allocated).
+ */
+void mg_strfree(struct mg_str *s);
+
 /*
  * Finds the first occurrence of a substring `needle` in the `haystack`.
  */