bolt12: fix "invalid currency UTF-8" test vector

The previous test vector incorrectly tested currency length encoding
rather than an invalid UTF-8 sequence in the currency field. This commit
updates the vector to have a correctly encoded length but an invalid
UTF-8 currency string, properly matching the test description.

Author: erickcestari

bolt12/offers-test.json

@@ -491,7 +491,7 @@
   {
     "description": "Malformed: invalid currency UTF-8",
     "valid": false,
-    "bolt12": "lno1qcpgqsg2q4q5cj2rg5tzzqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqg"
+    "bolt12": "lno1qcplllhapqpq86q2q4qkc6trv5tzzq6muh550qsfva9fdes0ruph7ctk2s8aqq06r4jxj3msc448wzwy9s"
   },
   {
     "description": "Malformed: truncated description UTF-8",
@@ -583,4 +583,4 @@
     "valid": false,
     "bolt12": "lno1pgx9getnwss8vetrw3hhyucsespjgef743p5fzqq9nqxh0ah7y87rzv3ud0eleps9kl2d5348hq2k8qzqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgqpqqqqqqqqqqqqqqqqqqqqqqqqqqqzqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqqzq3zyg3zyg3zygszqqqqyqqqqsqqvpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqszqgpqyqsq"
   }
-]
+]