Fix connector tool crash on overlapping shapes

• The connector built between two objects which overlap with each other in such a manner that neither contains the center of the other is an invisible connection. Which means, there's no curve for that connection at the moment, hence, it results into a crash when the cc_item_is_connector method tries to access the curve of the SPPath object.

• This commit adds guards to prevent null pointer dereferencing during such scenarios.

Fixes: #5314 (closed)

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