fix memory leak of %rep mmacro

[sorokin]

When running with -fsanitize=leak enabled nasm prints this error:

Direct leak of 960 byte(s) in 5 object(s) allocated from:
    #0 0x7f52b6464a37 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x55cf36676c46 in nasm_calloc nasmlib/alloc.c:72
    #2 0x55cf36676cd1 in nasm_zalloc nasmlib/alloc.c:87
    #3 0x55cf366e3980 in do_directive asm/preproc.c:4754
    #4 0x55cf366fec97 in pp_tokline asm/preproc.c:7773
    #5 0x55cf366ff84a in pp_getline asm/preproc.c:7837
    #6 0x55cf3667263c in assemble_file asm/nasm.c:1722
    #7 0x55cf3666b4e4 in main asm/nasm.c:719
    #8 0x7f52b5b7cd8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #9 0x7f52b5b7ce3f in __libc_start_main_impl ../csu/libc-start.c:392
    #10 0x55cf36666e04 in _start (/home/ivan/d/nasm/nasm+0x2e2e04)

This error is reproducible on lnxlinux.asm test or on this small snippet:

    %rep 8
        nop
        nop
        nop
    %endrep

The original call to free_mmacro was commented out in 91e72409bec0910456e1c34457a04ca00e2f7b99 as it caused use-after-free. https://bugzilla.nasm.us/show_bug.cgi?id=3392414

After adding free_mmacro I tested nasm with -fsanitize=address on all four reproducers attached to the issue and none of them causes use-after-free now. Also this commit passes all tests without causing use-after-free.

[hpax]

Is this still a problem?

[sorokin]

I can not reproduce this error on the latest commit 22a9118 in master.

At quick glance the bug might be fixed by the commit 9ea92ea. I suggest closing the issue.