Fix a double free in rz_mode when interrupting the repeat builtin.

==87167==ERROR: AddressSanitizer: attempting double-free on 0x602000001a70 in thread T0:
    #0 0x10d3c5fdd in wrap_free (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x56fdd)
    #1 0x10d2c52d2 in rz_mode main.c:127
    #2 0x10d2c2944 in doinput doit.c:27
    #3 0x10d2c4ea2 in main main.c:28
    #4 0x7fff7aa96014 in start (libdyld.dylib:x86_64+0x1014)

0x602000001a70 is located 0 bytes inside of 7-byte region [0x602000001a70,0x602000001a77)
freed by thread T0 here:
    #0 0x10d3c5fdd in wrap_free (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x56fdd)
    #1 0x10d2c8a11 in zrun zmodem.c:86
    #2 0x10d2c5259 in rz_mode main.c:123
    #3 0x10d2c2944 in doinput doit.c:27
    #4 0x10d2c4ea2 in main main.c:28
    #5 0x7fff7aa96014 in start (libdyld.dylib:x86_64+0x1014)

previously allocated by thread T0 here:
    #0 0x10d3c0952 in wrap_strdup (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x51952)
    #1 0x10d2c3dee in write_vector_word globbing.c:55
    #2 0x10d2c3b3c in glob_args globbing.c:35
    #3 0x10d2c889f in zparse zmodem.c:60
    #4 0x10d2c5240 in rz_mode main.c:121
    #5 0x10d2c2944 in doinput doit.c:27
    #6 0x10d2c4ea2 in main main.c:28
    #7 0x7fff7aa96014 in start (libdyld.dylib:x86_64+0x1014)

SUMMARY: AddressSanitizer: double-free (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x56fdd) in wrap_free
==87167==ABORTING

--- zmodem.c.orig	2003-09-02 03:02:15.000000000 +0000
+++ zmodem.c	2018-12-28 13:40:40.000000000 +0000
@@ -84,6 +84,7 @@
       if (gl_repeat && j)
       {
 	 free(av[0]);
+	 av[0] = NULL;
 	 av++;
       }
       j = 0;