[wasm] Fix signature canonicalization for error case.

The decoder should not attempt to insert null signatures into the SignatureMap.

R=ahaas@chromium.org

Bug: chromium:775366
Change-Id: I0fbc0547dbf00fd25d37271a03b6756481a4c6a1
Reviewed-on: https://chromium-review.googlesource.com/730752Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48793}