Made behavior of mg_get_cookie(..) regarding errors the same as of mg_get_var(..)

Fix for hang when sending large request with SSL enabled

When using SSL, we must use SSL_pending() in addition to/instead of select() to determine whether there is SSL data to be read.
This update makes the SSL_pending() function available and uses is to check for data if select() indicates there is no data.

See https://github.com/valenok/mongoose/issues/47

Made behavior of mg_get_cookie(..) on errors the same as mg_get_var(..). Separated the two cases a.) problems with destination buffer and b.) cookie header or cookie parameter not existing.