intervals: restrict check missing elements fix to sets with no auto-merge

intervals: restrict check missing elements fix to sets with no auto-merge

commit ce04d25b4a116ef04f27d0b71994f61a24114d6d upstream.

If auto-merge is enabled, skip check for element mismatch introduced by
6d1ee9267e7e ("intervals: check for EXPR_F_REMOVE in case of element
mismatch"), which is only relevant to sets with no auto-merge.

The interval adjustment routine for auto-merge already checks for
unexisting intervals in that case.

Uncovered via ASAN:

==11946==ERROR: AddressSanitizer: heap-use-after-free on address
0x60d00000021c at pc 0x559ae160d5b3 bp 0x7ffc37bcb800 sp 0x7ffc37bcb7f8
READ of size 4 at 0x60d00000021c thread T0
    #0 0x559ae160d5b2 in 0? /builddir/build/BUILD/nftables-1.0.6/src/intervals.c:424
    #1 0x559ae15cb05a in interval_set_eval.lto_priv.0 (/usr/lib64/libnftables.so.1+0xaf05a)
    #2 0x559ae15e1c0d in setelem_evaluate.lto_priv.0 (/usr/lib64/libnftables.so.1+0xc5c0d)
    #3 0x559ae166b715 in nft_evaluate (/usr/lib64/libnftables.so.1+0x14f715)
    #4 0x559ae16749b4 in nft_run_cmd_from_buffer (/usr/lib64/libnftables.so.1+0x1589b4)
    #5 0x559ae20c0e7e in main (/usr/bin/nft+0x8e7e)
    #6 0x559ae1341146 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #7 0x559ae1341204 in __libc_start_main_impl ../csu/libc-start.c:381
    #8 0x559ae20c1420 in _start ../sysdeps/x86_64/start.S:115

0x60d00000021c is located 60 bytes inside of 144-byte region [0x60d0000001e0,0x60d000000270) freed by thread T0 here:
    #0 0x559ae18ea618 in __interceptor_free ../../../../gcc-12.2.0/libsanitizer/asan/asan_malloc_linux.cpp:52
    #1 0x559ae160c315 in 4 /builddir/build/BUILD/nftables-1.0.6/src/intervals.c:349
    #2 0x559ae160c315 in 0? /builddir/build/BUILD/nftables-1.0.6/src/intervals.c:420

previously allocated by thread T0 here:
    #0 0x559ae18eb927 in __interceptor_calloc ../../../../gcc-12.2.0/libsanitizer/asan/asan_malloc_linux.cpp:77
    #1 0x559ae15c5076 in set_elem_expr_alloc (/usr/lib64/libnftables.so.1+0xa9076)

Fixes: 6d1ee9267e7e ("intervals: check for EXPR_F_REMOVE in case of element mismatch")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>