Zeroize internal buffers and variables in MD hashes

Zeroising of local buffers and variables which are used for calculations in
mbedtls_internal_md*_process() and mbedtls_internal_ripemd160_process()
functions to erase sensitive data from memory.
Checked all function for possible missing zeroisation in MD.

Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>

library/md2.c

@@ -177,6 +177,9 @@ int mbedtls_internal_md2_process( mbedtls_md2_context *ctx )
         t  = ctx->cksum[i];
     }
 
+    /* Zeroise variables to clear sensitive data from memory. */
+    mbedtls_platform_zeroize( &t, sizeof( t ) );
+
     return( 0 );
 }