Fix search_regs memory leak when thread destroyed

* src/thread.c (free_search_regs): New function.
(finalize_one_thread): Use it.

src/thread.c

@@ -768,9 +768,21 @@ run_thread (void *state)
   return NULL;
 }
 
+static void
+free_search_regs (struct re_registers *regs)
+{
+  if (regs->num_regs != 0)
+    {
+      xfree (regs->start);
+      xfree (regs->end);
+    }
+}
+
 void
 finalize_one_thread (struct thread_state *state)
 {
+  free_search_regs (&state->m_search_regs);
+  free_search_regs (&state->m_saved_search_regs);
   sys_cond_destroy (&state->thread_condvar);
 }