mpi: Avoid using freed pointer in mpi_lshift_limbs()

At the start of the function we assign 'a->d' to 'ap'. Then we use the
RESIZE_IF_NEEDED macro on 'a' - this may free 'a->d' and replace it
with newly allocaetd storage. In that case, we'll be operating on
freed memory further down in the function when we index into 'ap[]'.
Since we don't actually need 'ap' until after the use of the
RESIZE_IF_NEEDED macro we can just delay the assignment to it until
after we've potentially resized, thus avoiding the issue.

While I was there anyway I also changed the integer variable 'n' to be
const. It might as well be since we only assign to it once and use it
as a constant, and then the compiler will tell us if we ever assign to
it in the future.
Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

lib/mpi/mpi-bit.c

@@ -177,8 +177,8 @@ int mpi_rshift(MPI x, MPI a, unsigned n)
  */
 int mpi_lshift_limbs(MPI a, unsigned int count)
 {
-	mpi_ptr_t ap = a->d;
-	int n = a->nlimbs;
+	const int n = a->nlimbs;
+	mpi_ptr_t ap;
 	int i;
 
 	if (!count || !n)
@@ -187,6 +187,7 @@ int mpi_lshift_limbs(MPI a, unsigned int count)
 	if (RESIZE_IF_NEEDED(a, n + count) < 0)
 		return -ENOMEM;
 
+	ap = a->d;
 	for (i = n - 1; i >= 0; i--)
 		ap[i + count] = ap[i];
 	for (i = 0; i < count; i++)