avoid double free

If freedynamicdata() gets called twice, for whatever reason, a "double free" can occur.

warning: 44     ./nptl/pthread_kill.c: No such file or directory
(gdb) bt
#0  __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#3  0x00007ffff7c8b26e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007ffff7c6e8ff in __GI_abort () at ./stdlib/abort.c:79
#5  0x00007ffff7c6f7b6 in __libc_message_impl (fmt=fmt@entry=0x7ffff7e148d7 "%s\n")
    at ../sysdeps/posix/libc_fatal.c:132
#6  0x00007ffff7ceefe5 in malloc_printerr (str=str@entry=0x7ffff7e17bf0 "free(): double free detected in tcache 2")
    at ./malloc/malloc.c:5772
#7  0x00007ffff7cf154f in _int_free (av=0x7ffff7e49ac0 <main_arena>, p=<optimized out>, have_lock=0)
    at ./malloc/malloc.c:4541
#8  0x00007ffff7cf3d9e in __GI___libc_free (mem=0x555555ad82a0) at ./malloc/malloc.c:3398
#9  0x00005555557c12e9 in free_rect () at rect.c:48
#10 0x00005555557d77a2 in freedynamicdata () at save.c:1240
#11 0x0000555555682754 in nh_terminate (status=0) at end.c:1671
#12 0x000055555589af15 in opt_terminate () at ../sys/unix/unixmain.c:768
#13 0x000055555589af7a in after_opt_showpaths (dir=0x0) at ../sys/unix/unixmain.c:796
#14 0x0000555555693dd9 in do_deferred_showpaths (code=0) at files.c:4491
#15 0x0000555555778405 in initoptions () at options.c:6948
#16 0x0000555555899cd9 in main (argc=2, argv=0x7fffffffdad8) at ../sys/unix/unixmain.c:151

src/rect.c

@@ -46,6 +46,7 @@ free_rect(void)
 {
     if (rect)
         free(rect);
+    rect = 0;
     n_rects = rect_cnt = 0;
 }