Fix erts_check_off_heap2 assertion

The test needs to be false to fail, not true. This was noticed with the
following warning, where marking erl_assert_error as non-returning didn't
help:

beam/erl_gc.c:2772:2: warning: variable 'refc' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
        default:
        ^~~~~~~
beam/erl_gc.c:2775:26: note: uninitialized use occurs here
        ERTS_CHK_OFFHEAP_ASSERT(refc >= 1);
                                ^~~~
beam/erl_gc.c:2738:11: note: expanded from macro 'ERTS_CHK_OFFHEAP_ASSERT'
    if (!(EXP))                                         \
          ^
beam/erl_gc.c:2759:18: note: initialize the variable 'refc' to silence this warning
        erts_aint_t refc;
                        ^
                         = 0

0 files changed, 0 insertions, 0 deletions