api: fix panic in conn.NewWatcher()

[ImeevMA]

Before this patch, conn.c was not checked for nil before calling its method. This could cause a panic if the connection was lost or closed.

Closes #438

What has been done? Why? What problem is being solved?

I didn't forget about (remove if it is not applicable):

• [x] Tests (see documentation for a testing package)
• [x] Changelog (see documentation for changelog format)

Related issues: #438