[skjson] NaN while parsing long decimals

When parsing numbers with long decimals, the fast-path decimal factor
may collapse to zero.  Catch this condition (and defer to the slow parse
path).

Bug: skia:8499
Change-Id: I4e364402aecdcca9558d027a55ff297170e2a195
Reviewed-on: https://skia-review.googlesource.com/c/181179
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>