powerpc: Fix uninitialised variable in VSX alignment code

This fixes an uninitialised variable in the VSX alignment code.  It can
cause warnings from GCC (noticed with gcc-4.1.1).  Gcc is actually
correct in this instance, and this bug could cause the alignment
interrupt handler to send a SIGSEGV to the process on a legitimate
access.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>