Prevent memory leak when rejecting a call from on_incoming_call() callback

[riza]

The call->incoming_data() will get reset (set to NULL) on reset_call(),
which lead to memory leak since it's not properly released.

One example of this case is when rejecting a call from on_incoming_call() callback.

Thanks to Alexandre Beaulieu for the report.

[riza]

In 5584:

Fix #2012: Prevent memory leak when rejecting a call from on_incoming_call()
callback.

[riza]

In 5585:

Re #2012: Add additional comments.

[ming]

In 5592:

Re #2012: (minor) remove warning: '/*' within block comment