Dead signedness check if(incr<0) in sys_sbrk (incr is unsigned; latent, non-exploitable)

Summary

vm_unix.c:74 incr declared vm_offset_t (unsigned). :86 assigns uap->incr (size_t, unsigned). :94 if(incr<0) compares unsigned to 0 -> always false -> dead code. Intent (comment :88-93) is reject negative/shrink requests. Behavior still correct today only because subsequent overflow checks (:112,:116) reject negative intptr_t values. Risk: regression if bounds checks relaxed. libc passes intptr_t so negative values arrive as huge unsigned. Fix: if((intptr_t)uap->incr < 0) { error=EOPNOTSUPP; goto done; } before assigning to unsigned incr.