Make ssl_parse_serverhello_use_srtp_ext return 0 on failure.

Match most other functions. 03973096f416e694b676160ca481553bb44738eb changed
the call site, but not the function itself.

Change-Id: Iac018cbed8a454f50fb0da5e2419f503d8cb652d
Reviewed-on: https://boringssl-review.googlesource.com/1060
Reviewed-by: Adam Langley <agl@google.com>