Assert for negative field in bit32 polyfill

This more closely matches the behavior of lua5.2.

data/core/bit.lua

@@ -11,18 +11,23 @@ local function mask(n)
 	return (~((ALLONES << 1) << ((n) - 1)))
 end
 
+local function check_args(field, width)
+	assert(field >= 0, "field cannot be negative")
+	assert(width > 0, "width must be positive")
+	assert(field + width < LUA_NBITS and field + width >= 0,
+	       "trying to access non-existent bits")
+end
+
 function bit.extract(n, field, width)
 	local w = width or 1
-	assert(w > 0, "width must be positive")
-	assert(field + w < LUA_NBITS and field + w >= 0, "trying to access non-existent bits")
+	check_args(field, w)
 	local m = trim(n)
 	return m >> field & mask(w)
 end
 
 function bit.replace(n, v, field, width)
 	local w = width or 1
-	assert(w > 0, "width must be positive")
-	assert(field + w < LUA_NBITS and field + w >= 0, "trying to access non-existent bits")
+	check_args(field, w)
 	local m = trim(n)
 	local x = v & mask(width);
 	return m & ~(mask(w) << field) | (x << field)