Citeproc: Don't detect math elements as locators.

Closes #7321.

test/command/7321.md

@@ -0,0 +1,24 @@
+```
+% pandoc -t plain --citeproc --wrap=none
+---
+references:
+- id: fenner2012a
+  title: One-click science marketing
+  author:
+  - family: Fenner
+    given: Martin
+  container-title: Nature Materials
+  volume: 11
+  issue: 4
+  publisher: Nature Publishing Group
+  type: article-journal
+  issued:
+    year: 2012
+---
+
+[@fenner2012a, $a^2+b^2=c^2$]
+^D
+(Fenner 2012, a² + b² = c²)
+
+Fenner, Martin. 2012. “One-Click Science Marketing.” Nature Materials 11 (4).
+```