Fix false-positive vision estimation edgecase

[amquake]

Reported by @prensing

there is a problem if the found tags are a valid one plus an invalid tag, so eg it sees a tag but finds noise on the wall, and so lists tags 2 and 15. Lines 90-99 will ignore tag 15, and put 4 corners into the list. It will get past the test on line 100 (there are 4 corners). BUT then at line 106, there are 2 entries in visTags, so it will go to the "else" and try to use solvePNP_SQPNP().

[mcm001]

We ball