Ensure that a valid DPI is always set

(cherry picked from commit f19aa203c9)

src/kernel/qpaintdevice_x11.cpp

@@ -529,11 +529,16 @@ static void create_dpis()
     Q_CHECK_PTR( dpisX );
     Q_CHECK_PTR( dpisY );
     for ( i = 0; i < screens; i++ ) {
-	dpisX[ i ] = (DisplayWidth(dpy,i) * 254 + DisplayWidthMM(dpy,i)*5)
-
-		     / (DisplayWidthMM(dpy,i)*10);
-	dpisY[ i ] = (DisplayHeight(dpy,i) * 254 + DisplayHeightMM(dpy,i)*5)
-		     / (DisplayHeightMM(dpy,i)*10);
+       if (DisplayWidthMM(dpy,i) < 1)
+               dpisX[ i ] = 75; // default the dpi to 75.
+       else
+               dpisX[ i ] = (DisplayWidth(dpy,i) * 254 + DisplayWidthMM(dpy,i)*5)
+                     / (DisplayWidthMM(dpy,i)*10);
+       if (DisplayHeightMM(dpy,i) < 1)
+               dpisY[ i ] = 75; // default the dpi to 75.
+       else
+               dpisY[ i ] = (DisplayHeight(dpy,i) * 254 + DisplayHeightMM(dpy,i)*5)
+                     / (DisplayHeightMM(dpy,i)*10);
     }
 }