We went on decoding the next section, which happened to be the start
section. But since the function section had an error, the signature
pointer was not still {nullptr} on the start function, leading to a
segfault.

Drive-by fix: Improve decoder trace output.

R=ahaas@chromium.org
BUG=chromium:708714, chromium:708787

Change-Id: I5ae2adb32764b9d154f1ca878019f26ac31839b4
Reviewed-on: https://chromium-review.googlesource.com/472847Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44521}