Fix crash on template in svg or mathml namespace

lib/index.js

@@ -1,5 +1,5 @@
 /**
- * @import {ElementData, Element, Nodes, RootContent, Root} from 'hast'
+ * @import {ElementContent, ElementData, Element, Nodes, RootContent, Root} from 'hast'
  * @import {DefaultTreeAdapterMap, Token} from 'parse5'
  * @import {Schema} from 'property-information'
  * @import {Point, Position} from 'unist'
@@ -196,18 +196,30 @@ function element(state, node) {
   // Switch content.
   if (result.tagName === 'template') {
     const reference = /** @type {DefaultTreeAdapterMap['template']} */ (node)
-    const pos = reference.sourceCodeLocation
-    const startTag = pos && pos.startTag && position(pos.startTag)
-    const endTag = pos && pos.endTag && position(pos.endTag)
 
-    // Root in, root out.
-    const content = /** @type {Root} */ (one(state, reference.content))
+    // `template` in the SVG or MathML namespace does not get `content` from
+    // `parse5`, as that only applies to the HTML namespace: undo the
+    // `content` that `hastscript` always adds for `template`s.
+    if (reference.content) {
+      const pos = reference.sourceCodeLocation
+      const startTag = pos && pos.startTag && position(pos.startTag)
+      const endTag = pos && pos.endTag && position(pos.endTag)
 
-    if (startTag && endTag && state.file) {
-      content.position = {start: startTag.end, end: endTag.start}
-    }
+      // Root in, root out.
+      const content = /** @type {Root} */ (one(state, reference.content))
+
+      if (startTag && endTag && state.file) {
+        content.position = {start: startTag.end, end: endTag.start}
+      }
 
-    result.content = content
+      result.content = content
+    } else {
+      const placeholder = /** @type {Root} */ (result.content)
+      result.children = /** @type {Array<ElementContent>} */ (
+        placeholder.children
+      )
+      delete result.content
+    }
   }
 
   state.schema = schema

test/index.js

@@ -233,6 +233,51 @@ test('fromParse5', async function (t) {
     )
   })
 
+  await t.test(
+    'should not crash on a `template` in the svg or mathml namespace',
+    async function () {
+      assert.deepEqual(fromParse5(parseFragment('<svg><template>')), {
+        type: 'root',
+        children: [
+          {
+            type: 'element',
+            tagName: 'svg',
+            properties: {},
+            children: [
+              {
+                type: 'element',
+                tagName: 'template',
+                properties: {},
+                children: []
+              }
+            ]
+          }
+        ],
+        data: {quirksMode: false}
+      })
+
+      assert.deepEqual(fromParse5(parseFragment('<math><template>')), {
+        type: 'root',
+        children: [
+          {
+            type: 'element',
+            tagName: 'math',
+            properties: {},
+            children: [
+              {
+                type: 'element',
+                tagName: 'template',
+                properties: {},
+                children: []
+              }
+            ]
+          }
+        ],
+        data: {quirksMode: false}
+      })
+    }
+  )
+
   await t.test('should ignore prototypal props', async function () {
     assert.deepEqual(
       fromParse5(parseFragment('<x constructor y />'), {space: 'svg'}),