Fix #954

src/util.c

@@ -190,7 +190,7 @@ int mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap) {
     }
     // Microsoft version of vsnprintf() is not always null-terminated, so put
     // the terminator manually
-    (*buf)[len] = 0;
+    if (*buf != NULL) (*buf)[len] = 0;
     // LCOV_EXCL_STOP
   } else if (len >= (int) size) {
     /// Standard-compliant code path. Allocate a buffer that is large enough