main: also free memory on errors

* src/derives.c (derives_free): Beware of NULL.
* src/main.c (main): Let the 'finish' label include memory release.

src/derives.c

@@ -109,6 +109,9 @@ derives_compute (void)
 void
 derives_free (void)
 {
-  free (derives[0]);
-  free (derives);
+  if (derives)
+    {
+      free (derives[0]);
+      free (derives);
+    }
 }

src/main.c

@@ -207,6 +207,8 @@ main (int argc, char *argv[])
       timevar_pop (tv_parser);
     }
 
+ finish:
+
   timevar_push (tv_free);
   nullable_free ();
   derives_free ();
@@ -228,8 +230,6 @@ main (int argc, char *argv[])
   if (trace_flag & trace_bitsets)
     bitset_stats_dump (stderr);
 
- finish:
-
   /* Stop timing and print the times.  */
   timevar_stop (tv_total);
   timevar_print (stderr);