row-store can pass garbage keys to collator functions

The attached program collator_index_test.c sets up a collator on an index. During the second insertion, the collator compare is called with the expected key from the insertion, but with an unexpected key from the tree, a single null byte. The collator doesn't know what to make of this - it doesn't look like a (value,key) pair that can be decoded.

The caller of the callback is row_srch.c:294, which seems to be handling appends in a fast-path. If the fast-path code is commented out, it gives normal callbacks.

This test case is derived from attachment jcurs_u_extr_index_coll.c in WT-2308