Fix issue #5506 "WebServer serveStatic () can cause LoadProhibited exception in _svfprintf_r" (#5508)

"Using a Core Debug Level of Verbose and the WebServer serveStatic() function with the default value of nullptr for its cache_header argument, results in a LoadProhibited exception in _svfprintf_r().
This is because serveStatic() calls log_v() with cache_header corresponding to a "%s" in its format but without checking that cache_header is not nullptr, and then logv() (indirectly) calls _svfprintf_r().
On the other hand, with a Core Debug Level other than Verbose, this does not occur."

Changed serveStatic() to the check value of cache_header and if it is nullptr, instead pass an empty string to log_v().

Author: timr49

Diff for: libraries/WebServer/src/detail/RequestHandlersImpl.h

@@ -69,7 +69,7 @@ class StaticRequestHandler : public RequestHandler {
     , _cache_header(cache_header)
     {
         _isFile = fs.exists(path);
-        log_v("StaticRequestHandler: path=%s uri=%s isFile=%d, cache_header=%s\r\n", path, uri, _isFile, cache_header);
+        log_v("StaticRequestHandler: path=%s uri=%s isFile=%d, cache_header=%s\r\n", path, uri, _isFile, cache_header ? cache_header : ""); // issue 5506 - cache_header can be nullptr
         _baseUriLength = _uri.length();
     }