load_subtree(): check earlier whether an internal node is a tree entry

load_subtree(): check earlier whether an internal node is a tree entry

If an entry is not a tree entry, then it cannot possibly be an
internal node. But the old code checked this condition only after
allocating a leaf_node object and therefore leaked that memory.
Instead, check before even entering this branch of the code.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>