[LyX/master] XML: bad fix for a crash that only happened in XHTML.

[Thibaut Cuvelier]

commit faec7838222e5314ade3d89a3fadce731b93d759
Author: Thibaut Cuvelier <tcuvelier at lyx.org>
Date:   Fri Sep 11 00:39:41 2020 +0200

    XML: bad fix for a crash that only happened in XHTML.
    
    As an error is output, I suppose that this behaviour is OK.
---
 src/xml.cpp |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/src/xml.cpp b/src/xml.cpp
index 8e936f8..dda39a4 100644
--- a/src/xml.cpp
+++ b/src/xml.cpp
@@ -477,7 +477,8 @@ XMLStream &XMLStream::operator<<(xml::EndTag const &etag)
 						   + "' when other tags were pending. Last pending tag is `"
 						   + to_utf8(pending_tags_.back()->writeTag())
 						   + "'. Tag discarded.");
-				pending_tags_.erase(dit);
+				if (!pending_tags_.empty())
+					pending_tags_.erase(dit);
 				return *this;
 			}
 		}