Fix one byte too short malloc.

* src/engine-spawn.c (add_data): Fix malloc
--

Bummer.  Why did I subtracted one from the size?  Did I assume a
dynamically allocated structure with a string field which was not
going to be used?  Very strange.

Not a real problem though because malloc will anyway round up the
allocation to at least the next word size.

Detected by Stack 0.3.

src/engine-spawn.c

@@ -120,7 +120,7 @@ add_data (engine_spawn_t esp, gpgme_data_t data, int dup_to, int inbound)
   assert (esp);
   assert (data);
 
-  a = malloc (sizeof *a - 1);
+  a = malloc (sizeof *a);
   if (!a)
     return gpg_error_from_syserror ();
   a->next = NULL;