fix bug when using with unite

[987poiuytrewq]

When using the g:AutoPairsUseInsertedCount option with unite, auto-pairs crashes with the error:

Error detected while processing function <SNR>53_AutoPairsInsCntReset:
line    2:
E121: Undefined variable: b:autopairs_ins_cnt

This is when you leave insert mode from a unite buffer. I think this is because the unite buffer starts in insert mode, so the count doesn't get set.

This pull request checks if the count is set, defaulting to zero.

Is this the right place to put this pull request, or should it go in @dimonomid's repo https://github.com/dimonomid/auto-pairs-gentle ?