Fix some errors missing source positions

There was an off-by-one error in DebugHelpers.fileLine which meant we
never checked the first frame in the stack. This meant if we only had a
single Lua function on the stack (such as when in a fresh coroutine),
the error would not contain any line information.

src/main/java/org/squiddev/cobalt/debug/DebugHelpers.java

@@ -131,7 +131,7 @@ public static Buffer traceback(Buffer sb, LuaThread thread, int level) {
 	public static String fileLine(LuaThread thread) {
 		DebugState ds = thread.getDebugState();
 		DebugFrame di;
-		for (int i = 0, n = ds.top; i < n; i++) {
+		for (int i = 0, n = ds.top; i <= n; i++) {
 			di = ds.getFrame(i);
 			if (di != null && di.closure != null) {
 				return di.sourceLine();

src/test/resources/spec/vm_spec.lua

@@ -93,4 +93,16 @@ describe("The Lua VM", function()
 			expect("bar!"):eq(varA.hello)
 		end)
 	end)
+
+	describe("error positions", function()
+		it("includes positions when there is a single frame", function()
+			local function f() string.gsub(nil) end
+			local ok, err = coroutine.resume(coroutine.create(f))
+
+			local info = debug.getinfo(f, "S")
+			local prefix = info.short_src .. ":" .. info.linedefined .. ": bad argument"
+			expect(ok):eq(false)
+			expect(err:sub(1, #prefix)):eq(prefix)
+		end)
+	end)
 end)