Sanitize con and window rect dimensions

Make sure they're neither zero (prohibited by X11) nor overflown during
subtraction.

include/util.h

@@ -65,6 +65,7 @@ bool rect_contains(Rect rect, uint32_t x, uint32_t y);
 Rect rect_add(Rect a, Rect b);
 Rect rect_sub(Rect a, Rect b);
 bool rect_equals(Rect a, Rect b);
+Rect rect_sanitize_dimensions(Rect rect);
 
 /**
  * Returns true if the name consists of only digits.

src/render.c

@@ -64,6 +64,8 @@ void render_con(Con *con) {
         inset->width -= (2 * con->border_width);
         inset->height -= (2 * con->border_width);
 
+        *inset = rect_sanitize_dimensions(*inset);
+
         /* NB: We used to respect resize increment size hints for tiling
          * windows up until commit 0db93d9 here. However, since all terminal
          * emulators cope with ignoring the size hints in a better way than we
@@ -121,6 +123,8 @@ void render_con(Con *con) {
                 render_con_dockarea(con, child, &params);
             }
 
+            child->rect = rect_sanitize_dimensions(child->rect);
+
             DLOG("child at (%d, %d) with (%d x %d)\n",
                  child->rect.x, child->rect.y, child->rect.width, child->rect.height);
             x_raise_con(child);

src/util.c

@@ -53,6 +53,12 @@ Rect rect_sub(Rect a, Rect b) {
                   a.height - b.height};
 }
 
+Rect rect_sanitize_dimensions(Rect rect) {
+    rect.width = (int32_t)rect.width <= 0 ? 1 : rect.width;
+    rect.height = (int32_t)rect.height <= 0 ? 1 : rect.height;
+    return rect;
+}
+
 bool rect_equals(Rect a, Rect b) {
     return a.x == b.x && a.y == b.y && a.width == b.width && a.height == b.height;
 }