Fix wrong type argument when errors have no columns

[wyuenho]

For some checkers, sometimes the entire line can be just a single error and that error will not have any column info, in that case flycheck-inline-display-errors will raise a (wrong-type-argument number-or-marker-p nil) when trying to get the min of a list of nils. This PR fixes this issue.

@bbatsov

[mattiasdrp]

I'm encountering this issue regularly in elisp files and bash files. This is a simple change and it would greatly improve my workflow (I have to disable flycheck-inline from time to time to see the error at point)

[wyuenho]

Ping @bbatsov