libsframe: reuse static function sframe_decoder_get_funcdesc_at_index

sframe_decoder_get_funcdesc_at_index () is the function to access SFrame
FDEs in the SFrame decoder context.  Use it consistently.

Avoid unnecessary type cast and include minor enhancements as the code
is moved around.

libsframe/
	* sframe.c (sframe_decoder_get_funcdesc_at_index): Move some
	checks here.  Move the static function definition before the new
	use.
	(sframe_decoder_get_funcdesc): Use
	sframe_decoder_get_funcdesc_at_index instead.