ash: Don't crash associating touchscreens if there's no internal display
ash: Don't crash associating touchscreens if there's no internal display

This fixes a null pointer dereference in AssociateTouchscreens() since
45f6aaf ("ash: Support multiple touch input devices per display");

BUG=515201

Review URL: https://codereview.chromium.org/1270453002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#341107}

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