Remove redundant check

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/bignum_core.c b/library/bignum_core.c
index d5db4b0..bb266a6 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -201,7 +201,7 @@
 
         /* Avoid calling `memcpy` with NULL source or destination argument,
          * even if buflen is 0. */
-        if( buf != NULL && X != NULL )
+        if( buf != NULL )
         {
             Xp = (unsigned char*) X;
             memcpy( Xp + overhead, buf, buflen );