Fix pointer arithmetic error

app.cpp expands the pam environment to add XDG_SESSION_COOKIE from consolekit,
but the malloc isn't large enough to provide the final NULL element in the
array:

Example - Array starts with size of 4; 3 items + NULL.  Index 0-2 are items, 3
is NULL. The code increments n from 0 until NULL to get 3, add 1 more to get n=4
(the size of the original array).  To add an element, the malloc needs to be on
5 (n+1) rather than 4 (n) as it was before.  This patch addressed that.

The memcpy can copy 4 (ie, n), as there are 4 elements in the original array.
Also, index 3 (n-1) is the old NULL, where we want our new element, and 4 (n)
should be the new NULL, so those lines are correct and don't need adjustment.

app.cpp

@@ -589,7 +589,7 @@ void App::Login() {
 
         n++;
 
-        child_env = static_cast<char**>(malloc(sizeof(char*)*n));
+        child_env = static_cast<char**>(malloc(sizeof(char*)*(n+1)));
         memcpy(child_env, old_env, sizeof(char*)*n);
         child_env[n - 1] = StrConcat("XDG_SESSION_COOKIE=", ck.get_xdg_session_cookie());
         child_env[n] = NULL;