Remove bad overflow check from plat/osx/libsys/brk.c

Remove bad overflow check from plat/osx/libsys/brk.c

If I want to check for overflow, then I should check it before I do
base + incr, not after.

Now that I have no check, I am passing the overflowed base + incr to
brk1(), where it will probably fail the nbreak < segment check.