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.

2 files changed, 8 insertions, 4 deletions

diff --git a/src/w32term.c b/src/w32term.c
index 6d2fa335859..2785ae2b52d 100644
--- a/src/w32term.c
+++ b/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);
     }
 }
 
diff --git a/src/xterm.c b/src/xterm.c
index 2efa70b1dca..77daa22ae0d 100644
--- a/src/xterm.c
+++ b/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);
     }
 }