Fix decoding of chunked body.

Previously cowboy_http:te_chunked/2 would enter an incorrect state if
it tried to parse an incomplete chunk when the length was known from the
partial chunk.

Previosuly cowboy_http:te_chunked/2 expected the trailing "\r\n" to
always be present if chunk body was present in the buffer. This is not
guaranteed and so this commit accommodates that situation.