Prevent start_lost check when shifting out of translateable region

[aparton]

No description provided.

[coveralls]

Coverage increased (+0.0006%) to 83.032% when pulling 7823c71 on aparton:bugfix/shifting101 into 0e41a0d on Ensembl:release/101.

[sarahhunt]

I looks like there is something odd happening in _overlaps_start_codon and the variant should exit this routine earlier.
cdna_start_unshifted and cdna_end_unshifted are null, as the variant doesn't overlap the cDNA. The shifting offset is added to this, to give a cdna_start & cdna_end of 500, which overlaps the start codon. This addition would only be correct if we knew the the variant was at the base before the cDNA, which we don't.