Fix warning on unexpected paths

Paths like "comment/abc" (i.e., with a string in place of the comment ID) result in:

• Notice: Trying to get property of non-object in commentaccess_access_check() (line 393 of commentaccess.module).
• Notice: Trying to get property of non-object in commentaccess_access_check() (line 402 of commentaccess.module).
• Notice: Trying to get property of non-object in commentaccess_access_check() (line 405 of commentaccess.module).

Of course, this doesn't affect the module functionality, but the access checks should validate input.

This can be solved in several ways: for example, by testing that $comment->nid is valid just before using it, see attached patch.