malloc memory corruption on merge-tree with leading newline

["Stefan Frühwirth" <stefan.fruehwirth@uni-graz.at>]

Hi,

in one specific circumstance, git-merge-tree exits with a segfault 
caused by "*** Error in `git': malloc(): memory corruption (fast)":

There has to be at least one commit first (as far as I can tell it 
doesn't matter what content). Then create a tree containing a file with 
a leading newline character (\n) followed by some random string, and 
another tree with a file containing a string without leading newline. 
Now merge trees: Segmentation fault.

There is a test case[1] kindly provided by chrisrossi, which he crafted 
after I discovered the problem[2] in the context of Pylons/acidfs.

Best,
Stefan

[1] https://gist.github.com/chrisrossi/f09c8bed70b364f9f12e
[2] https://github.com/Pylons/acidfs/issues/3

For in-line reference, here's the test case:

git init bug
cd bug
echo b > a
git add a
git commit -m "first commit"
echo > b
echo -n a >> b
git add b
git commit -m "second commit, first branch"
git checkout HEAD~1
git checkout -b other
echo -n a > b
git add b
git commit -m "second commit, second branch"
git merge-tree HEAD~1 master other
cd ..
rm -rf bug