merge-recursive: be consistent with assert

merge-recursive: be consistent with assert

In commit 8daec1df03de ("merge-recursive: switch from (oid,mode) pairs
to a diff_filespec", 2019-04-05), an assertion on a->path && b->path
was added for code readability to document that these both needed to be
non-NULL at this point in the code.  However, the subsequent lines also
read o->path, so it should be included in the assert.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>