Fix silent crash when encountering internal stop codons. 

When calculating protein properties, Crawl will throw an unhandled exception when it encounters an internal stop codon in a protein sequence. This case is rare but does occur, e.g. with selenoproteins or simple annotation errors. However, it should not simply abort and return nothing (which leads to missing data on gene pages). This PR addresses this issue by removing internal stop codons in the protein sequence before calculating protein properties such as mass, isoelectric point or charge.