Fixes #4 node.getContext() is not a function

caused by unexpcted double colons in asciidoc content e.g. like this:

gRPC port:: 11004

packages/antora-xref-extension/lib/index.js

@@ -92,21 +92,25 @@ function register ({ config }) {
   }
 
   function processNode (src, node, lookup) {
-    const context = node.getContext()
-    if (context === 'paragraph' || context === 'admonition') {
-      const lines = node.lines
-      for (let i = 0; i < lines.length; i++) {
-        lines[i] = processLine(src, node, lines[i], lookup)
+    try {
+      const context = node.getContext()
+      if (context === 'paragraph' || context === 'admonition') {
+        const lines = node.lines
+        for (let i = 0; i < lines.length; i++) {
+          lines[i] = processLine(src, node, lines[i], lookup)
+        }
+      } else if (context === 'table') {
+        const rows = node.getRows()
+        rows.getHead().forEach((row) => row.forEach((cell) => processNode(src, cell, lookup)))
+        rows.getBody().forEach((row) => row.forEach((cell) => processNode(src, cell, lookup)))
+        rows.getFoot().forEach((row) => row.forEach((cell) => processNode(src, cell, lookup)))
+      } else if (context === 'table_cell' || context === 'list_item') {
+        node.text = processLine(src, node, node.text, lookup)
       }
-    } else if (context === 'table') {
-      const rows = node.getRows()
-      rows.getHead().forEach((row) => row.forEach((cell) => processNode(src, cell, lookup)))
-      rows.getBody().forEach((row) => row.forEach((cell) => processNode(src, cell, lookup)))
-      rows.getFoot().forEach((row) => row.forEach((cell) => processNode(src, cell, lookup)))
-    } else if (context === 'table_cell' || context === 'list_item') {
-      node.text = processLine(src, node, node.text, lookup)
+      node.getBlocks().forEach((child) => processNode(src, child, lookup))
+    } catch (t) {
+      log(node, 'warn', 'Parse error when validating xrefs. Check e.g. for double colons (\'::\')')
     }
-    node.getBlocks().forEach((child) => processNode(src, child, lookup))
   }
 
   function processLine (src, block, line, lookup) {

packages/antora-xref-extension/test/xref-fragments-extension-test.js

@@ -664,6 +664,27 @@ describe('xref extension', () => {
       expect(loggerDestination.messages).to.be.empty()
     })
 
+    it('should not fail but log a warning on unexpected syntax', () => {
+      const extensionConfig = {
+        logUnnecessaryLinkTextWarnings: false,
+      }
+      addFile(
+        'bad.adoc',
+        heredoc`
+      = Bad
+
+      bad::
+      `
+      )
+      run(extensionConfig)
+      const page = contentCatalog.getPages((candidate) => candidate.path === '/bad.adoc')[0]
+      expect(
+        loggerDestination.messages.some(
+          (message) => message.includes('"level":"warn"') && message.includes('Parse error when validating xrefs.')
+        )
+      ).to.be.true()
+    })
+
     function addFile (filename, contents) {
       contents = Buffer.from(contents)
       const mediaType = 'text/asciidoc'