internal/mcp/jsonschema: fix panic during schema validation

s.base is not yet set, so the fmt.Errorf call panics when accessing
s.base.uri.

Change-Id: I9e508406772d66bbf513bdbe2330c171df28ea85
Reviewed-on: https://go-review.googlesource.com/c/tools/+/684915
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>
Auto-Submit: Robert Findley <[email protected]>

Author: findleyr

internal/mcp/jsonschema/resolve.go

@@ -334,7 +334,7 @@ func resolveURIs(root *Schema, baseURI *url.URL) (map[string]*Schema, error) {
 			// The base URI for this schema is its $id resolved against the parent base.
 			s.uri = base.uri.ResolveReference(idURI)
 			if !s.uri.IsAbs() {
-				return fmt.Errorf("$id %s does not resolve to an absolute URI (base is %s)", s.ID, s.base.uri)
+				return fmt.Errorf("$id %s does not resolve to an absolute URI (base is %s)", s.ID, base.uri)
 			}
 			resolvedURIs[s.uri.String()] = s
 			base = s // needed for anchors