SSC: Test for overflow before init memory

Move 'memset(buf, 0, alloc_len)' into a helper function which
tests for sane alloc_len values (min(alloc_len, max_buffer_sz))

Signed-off-by: Mark Harvey <markh794@gmail.com>
SSC: Implement READ MEDIA SERIAL NUMBER

Signed-off-by: Mark Harvey <markh794@gmail.com>