fix Semi-AVID for files with holes

addresses #20 (closed)

this uses the supernova.mp3 test file from the issue, with less bytes and enough 0s at the front to introduce a null polynomial at the start

the fix

• iterate over the k polynomials instead of looking at the length of polynomials[0].coeffs() which is smaller than k in case the polynomial has holes in the higher monomials
• give a default value of 0_{\mathbb{F}_p} when trying to index into the polynomial coefficients