Fixed g++ compile error and warning

```
g++ -Wall -O2 -march=native -g -DLIBPATH='"/usr"' -DALTPATH='"/primus"' -DLIB64PATH='"/lib"' \ 
    -DLIB32PATH='"/lib32"' `pkg-config --cflags --libs gtk+-2.0` -o gtkglswitch gtkglswitch.cpp
gtkglswitch.cpp: In function 'int main(int, char**)':
gtkglswitch.cpp:132:21: error: taking address of temporary array
         NULL, &error))
                     ^
gtkglswitch.cpp:134:6: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]
   if (opt_default)
```