Avoid underflows in 'luaF_getlocalname'

A call like 'debug.getlocal(test, -2147483648)' could decrement
-2147483648, causing an underflow. (As this happens in a debug function,
the fix was not strictly necessary, but it has no cost in the generated
code.)

testes/db.lua

@@ -258,6 +258,7 @@ print'+'
 -- invalid levels in [gs]etlocal
 assert(not pcall(debug.getlocal, 20, 1))
 assert(not pcall(debug.setlocal, -1, 1, 10))
+assert(not debug.getlocal(test, -2147483648))
 
 
 -- parameter names