When a file upload ends in \r\n (i.e. a Windows formatted text file) the
sequence of bytes on the wire is `\r\n\r\n----...boundary-marker...`.

When the FSM in Webserver was evaluating the stream, that 2nd `\r` would
be written as valid data and the FSM parser would be reset and see `\n`
as the 1st character, which wouldn't match.  This would a) add an extra
`\r` to uploaded files, and b) cause uploads to hang.

Fix by checking on a header FSM mismatch if the next character input
could possibly match our marker, and if so handle it properly.

Fixes esp8266#7803