"git branch HEAD" dumps core when on detached head (NULL pointer dereference)

[Per Cederqvist <cederp@opera.com>]

Running "git branch HEAD" may be a stupid thing to do. It actually
was a mistake on my part. Still, I don't think git should dereference
a NULL pointer.

Tested using git 1.8.1.4 adn 1.8.1.1.

Repeat by:

     mkdir branchcrash || exit 1
     cd branchcrash
     git init
     touch a; git add a; git commit -m"Added a".
     touch b; git add b; git commit -m"Added b".
     git checkout HEAD^
     git branch HEAD

The last command dumps core.  gdb session:

gdb /usr/local/bin/git core
GNU gdb (Ubuntu/Linaro 7.4-2012.04-0ubuntu2.1) 7.4-2012.04
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /usr/local/bin/git...done.
[New LWP 7174]

warning: Can't read pathname for load map: Input/output error.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `git branch HEAD'.
Program terminated with signal 11, Segmentation fault.
#0  cmd_branch (argc=1, argv=0x7fffe6e2a0f0, prefix=<optimized out>)
     at builtin/branch.c:919
919			strbuf_addf(&buf, "refs/remotes/%s", branch->name);
(gdb) bt
#0  cmd_branch (argc=1, argv=0x7fffe6e2a0f0, prefix=<optimized out>)
     at builtin/branch.c:919
#1  0x00000000004046ac in run_builtin (argv=0x7fffe6e2a0f0, argc=2,
     p=<optimized out>) at git.c:273
#2  handle_internal_command (argc=2, argv=0x7fffe6e2a0f0) at git.c:434
#3  0x0000000000404df3 in run_argv (argv=0x7fffe6e29f90, 
argcp=0x7fffe6e29f9c)
     at git.c:480
#4  main (argc=2, argv=0x7fffe6e2a0f0) at git.c:555
(gdb) p branch
$1 = (struct branch *) 0x0
(gdb) quit

     /ceder