rtmpproto: Make sure to pass on the error code if read_connect failed

Previously, if read_connect failed, the ret variable was unmodified
and had the value 0, indicating success, which then was returned from
the rtmp_open function, even though it actually failed.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 6477139721f559b26eafd415e23e13ea2b0c27e1)