Fix memory leak detected in PR 1510.

LSAN/ASAN detected, on an error code path, that not all elements of a
struct ChannelCallFrame were freed.
channel.c: Fix for heap-use-after-free

ASAN detected this heap-use-after-free.
A job started by channel_from_job() could terminate and result in a call
to free_channel(), while channel_send_call() was still active/pending
and accessing Channel elements.

Original patch by @tarruda.
Merge pull request #1534 from oni-link/fix.leak.detected.in.1510

Fix memory leak detected in #1510.