Check `PrimaryFont()` in `NGInlinePaintContext::SetLineBox`

This patch adds a null-check for `Primaryfont()`. It's not
reproducible but clusterfuzz thinks they can.

Bug: 1385942
Change-Id: I355824aecaaa1dc7ce67ae98a08b5dfbdd01d286
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4048360
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1074668}