Better bounds check idIndex in ImGuiLayer

If the paint pointer references the upper half of memory the intptr_t
will be negative, which is not a value that is intended as an index into
fSkiaWidgetFuncs. Fixing this bounds check avoids a crash in viewer on
Android.

Change-Id: I0f90cca53d8ea01333bf1bedf5464aa2e074b9b4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1051338
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
Auto-Submit: Ben Wagner <bungeman@google.com>

Author: Ben Wagner

tools/viewer/ImGuiLayer.cpp

@@ -195,7 +195,7 @@ void ImGuiLayer::onPaint(SkSurface* surface) {
                 drawCmd->UserCallback(drawList, drawCmd);
             } else {
                 intptr_t idIndex = (intptr_t)drawCmd->TextureId;
-                if (idIndex < fSkiaWidgetFuncs.size()) {
+                if (0 <= idIndex && idIndex < fSkiaWidgetFuncs.size()) {
                     // Small image IDs are actually indices into a list of callbacks. We directly
                     // examing the vertex data to deduce the image rectangle, then reconfigure the
                     // canvas to be clipped and translated so that the callback code gets to use