tests: Check decode with high bits set fails

This doesn't catch out-of-bounds memory access, but it's a good test to have anyway.

tests/decode-highbit-prefix.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+hex=$(echo 993233 | xxd -r -p | base58 -d 25 || echo FAIL)
+test "x${hex}" = "xFAIL"

tests/decode-highbit.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+hex=$(echo 319932 | xxd -r -p | base58 -d 25 || echo FAIL)
+test "x${hex}" = "xFAIL"