utils_avltree.c: fix compiler warning

utils_avltree.c: fix compiler warning

make[3]: Entering directory '/home/ruben/src/collectd/src/daemon'
  CC       utils_avltree.lo
utils_avltree.c: In function ‘rebalance’:
utils_avltree.c:248:20: warning: logical ‘or’ of collectively exhaustive
tests is always true [-Wlogical-op]
    assert ((b_bottom >= -1) || (b_bottom <= 1));
                    ^~
utils_avltree.c:258:20: warning: logical ‘or’ of collectively exhaustive
tests is always true [-Wlogical-op]
    assert ((b_bottom >= -1) || (b_bottom <= 1));
                    ^~