unpack-tree.c: remove dead code

In line 1763 of unpack-tree.c we have a condition on the current tree
	if (current) {
		...
Within this block of code we can assume current to be non NULL, hence
the code after the statement in line 1796:
	if (current)
		return ...

cannot be reached.

The proposed patch here changes the order of the current tree and the
newtree part. I'm not sure if that's the right way to handle it.

All referenced lines have been introduced in the same commit
076b0adc (2006-07-30, read-tree: move merge functions to the library),
which was just moving the code around.
The outer condition on the current tree (now in line 1763) was introduced
in c859600954df4c292e, June 2005, [PATCH] read-tree: save more user hassles during fast-forward.
The inner condition on the current tree was introduced in
ee6566e8d70da682ac4926d, Sept. 2005, [PATCH] Rewrite read-tree

This issue was found by coverity, Id:290002

Signed-off-by: Stefan Beller <stefanbeller@gmail.com>
mailsplit.c: remove dead code

This was found by coverity. (Id: 290001)

the variable 'output' is only assigned to a value inequal to NUL,
after all gotos to the corrupt label.
Therefore we can conclude the two removed lines are actually dead code.

Signed-off-by: Stefan Beller <stefanbeller@gmail.com>