In relay_digest_matches(), use stack instead of heap.

We'd been using crypto_digest_dup() and crypto_digest_assign() here,
but they aren't necessary.  Instead we can just use the stack to
store the previous state of the SHA_CTX and avoid a malloc/free pair.

Closes ticket 24914.

changes/bug24914

@@ -0,0 +1,3 @@
+  o Minor features (performance):
+    - Avoid a needless call to malloc() when processing an incoming
+      relay cell.  Closes ticket 24914.