mbedtls_mpi_gcd: small optimization

Shifting TA and TB before the loop is not necessary. If A != 0, it will be
done at the start of the loop iteration. If A == 0, then lz==0 and G is
correctly set to B after 0 loop iterations.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index 2b41a14..acdccde 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2404,9 +2404,6 @@
     if( lzt < lz )
         lz = lzt;
 
-    MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &TA, lz ) );
-    MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &TB, lz ) );
-
     TA.s = TB.s = 1;
 
     while( mbedtls_mpi_cmp_int( &TA, 0 ) != 0 )