fix(emacs/lean-flycheck): fix bug in advice for lean-flycheck-try-parse-error-with-pattern

I provided an "advice" for 'flycheck-try-parse-error-with-pattern'
function to change its behavior, namely to increase error-columns by
one.

Bug
===

The problem is that it doesn't consider the case where the pattern is
not matched and ends up with "err = nil". For this case,
"(flycheck-error-column err)" generates an exception if executed. The
whole error parsing process stops immediately.

This causes a problem when we have more than one error-patterns, which
is the case when we enable 'error' and 'warning' patterns.

Fix
===

Fix is simple: check err before executing (flycheck-error-column err)

-            (col (flycheck-error-column err)))
+            (col (and err (flycheck-error-column err))))