Fix json_aivdm_dump for bad temp scaling

- Type 8, DAC = 1 FID = 11: airtemp, dewpoint and watertemp calculations go wrong due to unsigned ints.

Identified by: Stefan Roels <sroels-gpsd-dev@42solutions.nl>

unsigned/signed subtraction was causing math to go wonky.
new sample data added to sample.aivdm which exibits the conversion errors

Also identified watertemp that was failing from same data

Regenerated the corresponding check files from current code

1 files changed, 8 insertions, 0 deletions

diff --git a/test/sample.aivdm.ju.chk b/test/sample.aivdm.ju.chk
index 2e776357..d5c6fa96 100644
--- a/test/sample.aivdm.ju.chk
+++ b/test/sample.aivdm.ju.chk
@@ -76,3 +76,11 @@
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