util.ringbuffer: Prevent creation of buffer with negative size

Previously this would have been (unsigned)-1 which is a large positive
integer.

spec/util_ringbuffer_spec.lua

@@ -12,6 +12,11 @@ describe("util.ringbuffer", function ()
 				rb.new(0);
 			end);
 		end);
+		it("won't create a negatively sized buffer", function ()
+			assert.has_error(function ()
+				rb.new(-1);
+			end);
+		end);
 	end);
 	describe(":write", function ()
 		local b = rb.new();

util-src/ringbuffer.c

@@ -197,7 +197,7 @@ static int rb_free(lua_State *L) {
 }
 
 static int rb_new(lua_State *L) {
-	size_t size = luaL_optinteger(L, 1, sysconf(_SC_PAGESIZE));
+	lua_Integer size = luaL_optinteger(L, 1, sysconf(_SC_PAGESIZE));
 	luaL_argcheck(L, size > 0, 1, "positive integer expected");
 	ringbuffer *b = lua_newuserdata(L, sizeof(ringbuffer) + size);