cred-encoding: Avoid potential use after free when caching encodings

The pattern currently is to call get_cache(), generate the encoding
if that failed and then store it with cache().  The latter adopts the
passed encoding and frees any stored encoding.  However, the latter means
that if two threads concurrently fail to get a cached encoding and then
both generate and store one, one of the threads might use an encoding
that was freed by the other thread.

Since encodings are not expected to change, we can avoid this issue by
not replacing an existing cache entry and instead return that (while
freeing the passed value instead of the cached one).

Closes strongswan/strongswan#1231

src/libstrongswan/credentials/cred_encoding.h

@@ -202,14 +202,16 @@ struct cred_encoding_t {
 	 * Cache a credential encoding created externally.
 	 *
 	 * After calling cache(), the passed encoding is owned by the cred encoding
-	 * facility.
+	 * facility.  Note that if there already is a cached encoding for the same
+	 * key, the method will return that and free the passed value.
 	 *
 	 * @param type			format of the credential encoding
 	 * @param cache			key to use for caching, as given to encode()
-	 * @param encoding		encoding to cache, gets owned by this
+	 * @param encoding		encoding to cache, gets adopted, might get replaced
+	 *						with cached value
 	 */
 	void (*cache)(cred_encoding_t *this, cred_encoding_type_t type, void *cache,
-				  chunk_t encoding);
+				  chunk_t *encoding);
 
 	/**
 	 * Register a credential encoder function.