The result of ngx_rbtree_min() is always a node with the left child equal to
sentinel, thus the check is unnecessary.

It's mostly dead code.  And the idea of thread support for this task has
been deprecated.

Integer overflow is undefined behaviour in C and this indeed caused
problems on Solaris/SPARC (at least in some cases).  Fix is to
subtract unsigned integers instead, and then cast result to a signed
one, which is implementation-defined behaviour and used to work.

Strictly speaking, we should compare (unsigned) result with the maximum
value of the corresponding signed integer type instead, this will be
defined behaviour.  This will require much more changes though, and
considered to be overkill for now.