http: Avoid undefined behavior in va_arg conversion

Subtracting a null pointer is undefined behavior according to
Clang 13 (-Wnull-pointer-subtraction). Avoid this by casting to
intptr_t instead.

Request macros (e.g. OP_GET_SERVER_INFO) already do a simple cast
`((char*)_request)` rather than addition with null pointer
`(_request+(char*)NULL)` so nothing needs to be changed on that end.

src/http.c

@@ -3466,10 +3466,12 @@ static void *op_url_stream_vcreate_impl(OpusFileCallbacks *_cb,
   pinfo=NULL;
   *_pinfo=NULL;
   for(;;){
-    ptrdiff_t request;
-    request=va_arg(_ap,char *)-(char *)NULL;
+    char *prequest;
+    intptr_t request;
+    prequest=va_arg(_ap,char *);
     /*If we hit NULL, we're done processing options.*/
-    if(!request)break;
+    if(!prequest)break;
+    request=(intptr_t)prequest;
     switch(request){
       case OP_SSL_SKIP_CERTIFICATE_CHECK_REQUEST:{
         skip_certificate_check=!!va_arg(_ap,opus_int32);