alpha: fix handling of a3 on straced syscalls

For successful syscall that happens to return a negative, we want
a3 set to 0, no matter whether it's straced or not.  As it is,
for straced case we leave the value it used to have on syscall
entry.  Easily fixed, fortunately...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>