pool: fix compiler warning

pool: fix compiler warning

clang-4.0 reported:

util-pool.c:242:13: warning: logical not is only applied to the left hand side of this bitwise operator [-Wlogical-not-parentheses]
        if (! pb->flags & POOL_BUCKET_PREALLOCATED) {
            ^           ~
util-pool.c:242:13: note: add parentheses after the '!' to evaluate the bitwise operator first
        if (! pb->flags & POOL_BUCKET_PREALLOCATED) {
            ^
              (                                   )
util-pool.c:242:13: note: add parentheses around left hand side expression to silence this warning
        if (! pb->flags & POOL_BUCKET_PREALLOCATED) {
            ^
            (          )
util-pool.c:261:13: warning: logical not is only applied to the left hand side of this bitwise operator [-Wlogical-not-parentheses]
        if (! pb->flags & POOL_BUCKET_PREALLOCATED) {
            ^           ~
util-pool.c:261:13: note: add parentheses after the '!' to evaluate the bitwise operator first
        if (! pb->flags & POOL_BUCKET_PREALLOCATED) {
            ^
              (                                   )
util-pool.c:261:13: note: add parentheses around left hand side expression to silence this warning
        if (! pb->flags & POOL_BUCKET_PREALLOCATED) {
            ^
            (          )
2 warnings generated.