Work around memory-aliasing in descriptor ParsePubkey

Author: MarcoFalke

src/script/descriptor.cpp

@@ -826,7 +826,8 @@ std::unique_ptr<PubkeyProvider> ParsePubkey(uint32_t key_exp_index, const Span<c
     }
     if (origin_split.size() == 1) return ParsePubkeyInner(key_exp_index, origin_split[0], permit_uncompressed, out, error);
     if (origin_split[0].size() < 1 || origin_split[0][0] != '[') {
-        error = strprintf("Key origin start '[ character expected but not found, got '%c' instead", origin_split[0][0]);
+        error = strprintf("Key origin start '[ character expected but not found, got '%c' instead",
+                          origin_split[0].size() < 1 ? /** empty, implies split char */ ']' : origin_split[0][0]);
         return nullptr;
     }
     auto slash_split = Split(origin_split[0].subspan(1), '/');