[LyX/master] Fix #10649

[Juergen Spitzmueller]

commit 6fa457dddeea1693d32b3a43e97466506c9326f3
Author: Juergen Spitzmueller <spitz at lyx.org>
Date:   Tue Feb 15 10:24:39 2022 +0100

    Fix #10649
---
 src/insets/InsetIndex.cpp |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/src/insets/InsetIndex.cpp b/src/insets/InsetIndex.cpp
index fef2467..3cce6af 100644
--- a/src/insets/InsetIndex.cpp
+++ b/src/insets/InsetIndex.cpp
@@ -161,8 +161,10 @@ void InsetIndex::latex(otexstream & ios, OutputParams const & runparams_in) cons
 														   "explained in the User Guide."), spart));
 			}
 			// remove remaining \'s for the sorting part
-			docstring const ppart =
+			docstring ppart =
 					subst(spart_latexed.first, from_ascii("\\"), docstring());
+			// Plain quotes need to be escaped, however (#10649)
+			ppart = subst(ppart, from_ascii("\""), from_ascii("\\\""));
 			os << ppart;
 			os << '@';
 		}