Serious bug fix in entropy.c

Bug: mutex access within entropy_contex after it has been zeroed leads to app crash.
diff --git a/library/entropy.c b/library/entropy.c
index c90c7e4..540a27c 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -94,10 +94,10 @@
 #if defined(POLARSSL_HAVEGE_C)
     havege_free( &ctx->havege_data );
 #endif
-    polarssl_zeroize( ctx, sizeof( entropy_context ) );
 #if defined(POLARSSL_THREADING_C)
     polarssl_mutex_free( &ctx->mutex );
 #endif
+    polarssl_zeroize( ctx, sizeof( entropy_context ) );
 }
 
 int entropy_add_source( entropy_context *ctx,