bug#23640: 25.1.50; Getting rid of compiler warnings

[Ken Brown]

Building master on Cygwin with gcc-5.3.0 yields many "control reaches
end of non-void function" warnings.  What's the best way to get rid of
these?  Should I add return statements that will never be reached, like
this?

--- a/src/frame.c
+++ b/src/frame.c
@@ -263,6 +263,8 @@ See also `frame-live-p'.  */)
       return Qns;
     default:
       emacs_abort ();
+      /* Pacify compiler.  */
+      return Qnil;
     }
 }

Ken