Fix to allow debug.upvalue() to set upvalues to nil

[bruce-hill]

This should make it faster (it now stops scanning when it hits the desired upvalue) and more correct. It now allows setting an upvalue to a falsey or nil value, e.g. debug.upvalue(fn, 'foo', nil) (set 'foo' to nil), which is treated differently different from debug.upvalue(fn, 'foo') (get the value of 'foo').