main/app.c: Memory corruption from early format destruction.

* make_silence() created a malloced silence slin frame without adding a
slin format ref.  When the frame is destroyed it will unref the slin
format that never had a ref added.  Memory corruption is expected to
follow.

* Simplified and fixed counting the number of samples in a frame list for
make_silence().

* Eliminated an unnecessary RAII_VAR associated with the make_silence()
frame.

Change-Id: I47de3f9b92635b7f8b4d72309444d6c0aee6f747