Citeproc: Don't detect math elements as locators.

Closes #7321.

MANUAL.txt

@@ -5015,7 +5015,7 @@ the suffix as locator by prepending curly braces:
 
     [@smith{ii, A, D-Z}, with a suffix]
     [@smith, {pp. iv, vi-xi, (xv)-(xvii)} with suffix here]
-    [@smith{}, $a^2$ and following]
+    [@smith{}, 99 years later]
 
 A minus sign (`-`) before the `@` will suppress mention of
 the author in the citation.  This can be useful when the

src/Text/Pandoc/Citeproc/Locator.hs

@@ -181,6 +181,7 @@ pPageUnit = roman <|> plainUnit
       plainUnit = do
           ts <- many1 (notFollowedBy pSpace >>
                        notFollowedBy pLocatorPunct >>
+                       notFollowedBy pMath >>
                        anyToken)
           let s = stringify ts
           -- otherwise look for actual digits or -s
@@ -211,6 +212,12 @@ pMatchChar msg f = satisfyTok f' <?> msg
 pSpace :: LocatorParser Inline
 pSpace = satisfyTok (\t -> isSpacey t || t == Str "\160") <?> "space"
 
+pMath :: LocatorParser Inline
+pMath = satisfyTok isMath
+ where
+  isMath (Math{}) = True
+  isMath _ = False
+
 satisfyTok :: (Inline -> Bool) -> LocatorParser Inline
 satisfyTok f = tokenPrim show (\sp _ _ -> sp) (\tok -> if f tok
                                                           then Just tok

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).
+```