sparse-index: fix uninitialized jump

sparse-index: fix uninitialized jump

While testing the sparse-index, I verified a test with --valgrind and it
complained about an uninitialized value being used in a jump in the
path_matches_pattern_list() method. The line was this one:

	if (*dtype == DT_UNKNOWN)

In the call stack, the culprit was the initialization of the dtype
variable in convert_to_sparse_rec().

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>