Fix mpi_bigendian_to_host() on bigendian systems

The previous implementation of mpi_bigendian_to_host() did
a byte-swapping regardless of the endianness of the system.

Fixes #2622.

library/bignum.c

@@ -742,10 +742,15 @@ int mbedtls_mpi_write_file( const char *p, const mbedtls_mpi *X, int radix, FILE
 static mbedtls_mpi_uint mpi_uint_bigendian_to_host_c( mbedtls_mpi_uint x )
 {
     uint8_t i;
+    unsigned char *x_ptr;
     mbedtls_mpi_uint tmp = 0;
-    /* This works regardless of the endianness. */
-    for( i = 0; i < ciL; i++, x >>= 8 )
-        tmp |= ( x & 0xFF ) << ( ( ciL - 1 - i ) << 3 );
+
+    for( i = 0, x_ptr = (unsigned char*) &x; i < ciL; i++, x_ptr++ )
+    {
+        tmp <<= CHAR_BIT;
+        tmp |= (mbedtls_mpi_uint) *x_ptr;
+    }
+
     return( tmp );
 }