libmbedtls: mbedtls_mpi_shrink(): fix possible unwanted truncation

If mbedtls_mpi_shrink() is passed a value for nblimbs that is smaller
than the minimum number of limbs required to store the big number, the
current implementation will unexpectedly truncate the number to the
requested size. It should use the minimal size instead in order not to
corrupt the bigum value.

This issue was introduced in [1] probably as a result of a bad copy
and paste from mbedtls_mpi_grow().

Fixes: [1] commit 98bd5fe ("libmbedtls: add mbedtls_mpi_init_mempool()")
Reported-by: Zhenke Ma <zhenke.ma@armchina.com>
Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

lib/libmbedtls/mbedtls/library/bignum.c

@@ -206,14 +206,14 @@ int mbedtls_mpi_shrink( mbedtls_mpi *X, size_t nblimbs )
 
     if( X->use_mempool )
     {
-        p = mempool_alloc( mbedtls_mpi_mempool, nblimbs * ciL );
+        p = mempool_alloc( mbedtls_mpi_mempool, i * ciL );
         if( p == NULL )
             return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
-        memset( p, 0, nblimbs * ciL );
+        memset( p, 0, i * ciL );
     }
     else
     {
-        p = (mbedtls_mpi_uint*)mbedtls_calloc( nblimbs, ciL );
+        p = (mbedtls_mpi_uint*)mbedtls_calloc( i, ciL );
         if( p == NULL )
             return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
     }