Fix strtoul error condition (#1995)

endptr is set to '\0' when successful

code/espurna/utils.ino

@@ -195,7 +195,7 @@ namespace Heartbeat {
         // invalidate the whole string when invalid chars are detected
         char *value_endptr = nullptr;
         const auto value = strtoul(cfg.c_str(), &value_endptr, 10);
-        if (value_endptr) {
+        if (value_endptr == cfg.c_str() || value_endptr[0] != '\0') {
             return defaultValue();
         }