luci-app-nlbw: fix sporadic premature EOF when rendering JSON data

Specific timing patterns sometimes caused the LuCI controller to prematurely
stop reading data, resulting in truncated JSON output.

Turn the nonblocking waitpid() call into a blocking call after the IO read
loop to avoid this issue.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

applications/luci-app-nlbwmon/luasrc/controller/nlbw.lua

@@ -28,16 +28,17 @@ local function exec(cmd, args, writer)
 
 		while true do
 			local buffer = fdi:read(2048)
-			local wpid, stat, code = nixio.waitpid(pid, "nohang")
 
-			if writer and buffer and #buffer > 0 then
-				writer(buffer)
-			end
-
-			if wpid and stat == "exited" then
+			if not buffer or #buffer == 0 then
 				break
 			end
+
+			if writer then
+				writer(buffer)
+			end
 		end
+
+		nixio.waitpid(pid)
 	elseif pid == 0 then
 		nixio.dup(fdo, nixio.stdout)
 		fdi:close()