Avoid floating-point exceptions while drawing underwave

* src/w32term.c (x_get_scale_factor):
* src/xterm.c (x_get_scale_factor): Don't let the scale factors
become less than 1.  Reported by Yuri D'Elia <wavexx@thregr.org> in
http://lists.gnu.org/archive/html/emacs-devel/2017-08/msg00459.html.

src/w32term.c

@@ -318,8 +318,10 @@ x_get_scale_factor(struct w32_display_info *dpyinfo, int *scale_x, int *scale_y)
 
   if (dpyinfo)
     {
-      *scale_x = floor (dpyinfo->resx / base_res);
-      *scale_y = floor (dpyinfo->resy / base_res);
+      if (dpyinfo->resx > base_res)
+	*scale_x = floor (dpyinfo->resx / base_res);
+      if (dpyinfo->resy > base_res)
+	*scale_y = floor (dpyinfo->resy / base_res);
     }
 }
 

src/xterm.c

@@ -3483,8 +3483,10 @@ x_get_scale_factor(Display *disp, int *scale_x, int *scale_y)
 
   if (dpyinfo)
     {
-      *scale_x = floor (dpyinfo->resx / base_res);
-      *scale_y = floor (dpyinfo->resy / base_res);
+      if (dpyinfo->resx > base_res)
+	*scale_x = floor (dpyinfo->resx / base_res);
+      if (dpyinfo->resy > base_res)
+	*scale_y = floor (dpyinfo->resy / base_res);
     }
 }