luci-base: cbi.lua: fix TypedSection.parse()

It's not necessary to execute all code if section already exists.

Signed-off-by: Darius Joksas <jok.darius@gmail.com>
[reword commit message, squash commits, remove stray semicolon]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/luasrc/cbi.lua

@@ -1199,9 +1199,9 @@ function TypedSection.parse(self, novld)
 			if name then
 				-- Ignore if it already exists
 				if self:cfgvalue(name) then
-					name = nil;
-				end
-
+					name = nil
+					self.err_invalid = true
+				else
 					name = self:checkscope(name)
 
 					if not name then
@@ -1216,6 +1216,7 @@ function TypedSection.parse(self, novld)
 					end
 				end
 			end
+		end
 
 		if created then
 			AbstractSection.parse_optionals(self, created)