multi: Change curl_multi_wait/poll to error on negative timeout

- Add new error CURLM_BAD_FUNCTION_ARGUMENT and return that error when
  curl_multi_wait/poll is passed timeout param < 0.

Prior to this change passing a negative value to curl_multi_wait/poll
such as -1 could cause the function to wait forever.

Reported-by: hamstergene@users.noreply.github.com

Fixes https://github.com/curl/curl/issues/4763

Closes https://github.com/curl/curl/pull/4765

include/curl/multi.h

@@ -72,7 +72,8 @@ typedef enum {
                             attempted to get added - again */
   CURLM_RECURSIVE_API_CALL, /* an api function was called from inside a
                                callback */
-  CURLM_WAKEUP_FAILURE, /* wakeup is unavailable or failed */
+  CURLM_WAKEUP_FAILURE,  /* wakeup is unavailable or failed */
+  CURLM_BAD_FUNCTION_ARGUMENT,  /* function called with a bad parameter */
   CURLM_LAST
 } CURLMcode;