Fix bug: 'src_image' might be NULL if width||height are 0 (#3188223)

With ROT90-SystemTray fluxbox crashed. It is a bit unclear of where
to catch pixmaps / windows with either width or height equal to 0; IMHO
this needs more investigation.

src/FbTk/FbPixmap.cc

@@ -218,14 +218,21 @@ void FbPixmap::rotate(FbTk::Orientation orient) {
     unsigned int neww = oldw, newh = oldh;
     translateSize(orient, neww, newh);
 
+    // reverse height/width for new pixmap
+    FbPixmap new_pm(drawable(), neww, newh, depth());
+
+    // width|height could be 0. this happens (for example) if
+    // the systemtray-tool is ROT90. in that case 'src_image'
+    // becomes NULL and caused a SIGSEV upon XDestroyImage()
+    // TODO: catch dimensions with '0' earlier?
+    //
     // make an image copy
     XImage *src_image = XGetImage(display(), drawable(),
                                   0, 0, // pos
                                   oldw, oldh, // size
                                   ~0, // plane mask
                                   ZPixmap); // format
-    // reverse height/width for new pixmap
+    if (src_image) {
-    FbPixmap new_pm(drawable(), neww, newh, depth());
 
         GContext gc(drawable());
 
@@ -273,6 +280,8 @@ void FbPixmap::rotate(FbTk::Orientation orient) {
         }
 
         XDestroyImage(src_image);
+    }
+
     // free old pixmap and set new from new_pm
     free();