properly check for "all good sigs are weak"

properly check for "all good sigs are weak"

Using erase(pos) is invalid in our case here as pos must be a valid and
derefenceable iterator, which isn't the case for an end-iterator (like
if we had no good signature).
The problem runs deeper still through as VALIDSIG is a keyid while
GOODSIG is just a longid so comparing them will always fail.

Closes: 818910