Coverity: fix memory leak on error in test function.

The function make_intro_from_plaintext() in test_introduce.c would
leak memory if we ever hit a failure from our underlying crypto
functions.  This kind of failure should be impossible, but it's best
to be safe here.

Bugfix on 0.2.4.1-alpha.

src/test/test_introduce.c

@@ -383,8 +383,10 @@ make_intro_from_plaintext(
 
   /* Output the cell */
   *cell_out = cell;
+  cell = NULL;
 
  done:
+  tor_free(cell);
   return cell_len;
 }
 
@@ -535,4 +537,3 @@ struct testcase_t introduce_tests[] = {
   INTRODUCE_LEGACY(late_parse_v3),
   END_OF_TESTCASES
 };
-