Segfaults in sha256

[kapouer]

On node 0.4.12, v8 3.1 :

#3  0x00007ffff5a32606 in malloc_printerr (action=3, str=0x7ffff5b062ca "free(): invalid pointer", ptr=<optimized out>) at alloc.c:6283
#4  0x00007ffff5a3733c in *__GI___libc_free (mem=<optimized out>) at malloc.c:3738
#5  0x00007fffcfee5abe in sha256 (args=...) at ../hashlib.cc:215
#6  0x00007ffff682cfe5 in ?? () from /usr/lib/libv8.so.3.1.8.22

On node 0.4.12, v8 3.4 or 3.5 :

#1  0x00007ffff7ff28b3 in v8::HandleScope::Close<v8::String> (this=0x7fffffffd7a0, value=...) at /usr/include/v8.h:3951
#2  0x00007ffff7ff0a8d in sha256 (args=...) at ../hashlib.cc:226

I had to make very small changes to the second test so node works with libv8 3.4, then with 3.5 branches.