signature: Small signature optimization

[tgonzalezorlandoarm]

Only calculate the signature when necessary. When sigsize is smaller than the expected signature length, preemptively exit the function instead of doing that after having calculated the signature.

This saves some computation power.

[tgonzalezorlandoarm]

This one comes in in replacement of #46 . For the future, perhaps we can find a way of making use of a standard get_signature_len function