RST reader: fix bug with internal link targets.

They were gobbling up indented content underneath.
Closes #4919.

src/Text/Pandoc/Readers/RST.hs

@@ -1090,7 +1090,7 @@ referenceKey = do
 targetURI :: Monad m => ParserT [Char] st m [Char]
 targetURI = do
   skipSpaces
-  optional newline
+  optional $ try $ newline >> notFollowedBy blankline
   contents <- trim <$>
      many1 (satisfy (/='\n')
      <|> try (newline >> many1 spaceChar >> noneOf " \t\n"))

test/command/4919.md

@@ -0,0 +1,14 @@
+```
+% pandoc -f rst -t native
+.. _`tgtmath`:
+
+  .. math::
+     :name:
+
+     V = \frac{K}{r^2}
+^D
+[Div ("tgtmath",[],[])
+ [BlockQuote
+  [Para [Math DisplayMath "V = \\frac{K}{r^2}"]]]]
+```
+