fix memory leak in elf_init

[sorokin]

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

Direct leak of 31 byte(s) in 1 object(s) allocated from:
    #0 0x7faadfc56867 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x7faadf3ecaac in __GI___libc_scratch_buffer_dupfree malloc/scratch_buffer_dupfree.c:32
    #2 0x7faadf395eed in scratch_buffer_dupfree ../include/scratch_buffer.h:147
    #3 0x7faadf395eed in realpath_stk stdlib/canonicalize.c:424
    #4 0x7faadf3964f5 in __GI___realpath stdlib/canonicalize.c:446
    #5 0x7faadfbe6b7c in __interceptor_canonicalize_file_name ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:3682
    #6 0x5643cfb1b058 in nasm_realpath nasmlib/realpath.c:58
    #7 0x5643cfaaaef5 in elf_init output/outelf.c:547
    #8 0x5643cfaaade7 in elf32_init output/outelf.c:496
    #9 0x5643cf9d1454 in main asm/nasm.c:716
    #10 0x7faadf36ed8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #11 0x7faadf36ee3f in __libc_start_main_impl ../csu/libc-start.c:392
    #12 0x5643cf9cce04 in _start (/home/ivan/d/nasm/nasm+0x2e2e04)

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x7faadfc56867 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x5643cf9dcbc4 in nasm_malloc nasmlib/alloc.c:55
    #2 0x5643cf9dced4 in nasm_strndup nasmlib/alloc.c:127
    #3 0x5643cfb1b4e4 in nasm_dirname nasmlib/path.c:152
    #4 0x5643cfaaaf4d in elf_init output/outelf.c:550
    #5 0x5643cfaaade7 in elf32_init output/outelf.c:496
    #6 0x5643cf9d1454 in main asm/nasm.c:716
    #7 0x7faadf36ed8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #8 0x7faadf36ee3f in __libc_start_main_impl ../csu/libc-start.c:392
    #9 0x5643cf9cce04 in _start (/home/ivan/d/nasm/nasm+0x2e2e04)

This error is reproducible on any test that is run with -felf.

The problem is fixed by calling nasm_free appropriately. I had to remove const from pointers because I get -Wdiscarded-qualifiers warning otherwise, because nasm_free takes non-const pointer.