libopencore-amr: Check the return value of amr_decode_fix_avctx

This allows getting rid of redundant checks later in the codec
specific init functions.

Move the check to before actually initializing the decoder lib,
to simplify error handling.

This fixes a case of returning a value from a void function, present since
d40dab907.

Signed-off-by: Martin Storsjö <martin@martin.st>
