[LyX/master] Fix crash due to encoding issues with child doc

[Scott Kostyshak]

commit c61472303e4ea1d59d1f26e9d4a516dd2c50feb2
Author: Scott Kostyshak <skostysh at lyx.org>
Date:   Sun Dec 8 19:14:38 2019 -0500

    Fix crash due to encoding issues with child doc
    
    This crash occurred starting with 553bebc3, and can be triggered
    when preview is enabled both in preferences and in a child document
    inset. For a minimal example, see the following ML thread:
    
      https://www.mail-archive.com/search?l=mid&q=20191209002609.6fao3dljtf3ohl25%40tallinn
    
    This fix restores behavior to before that commit for the case when
    oldEnc is 0.
---
 src/insets/InsetInclude.cpp |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/insets/InsetInclude.cpp b/src/insets/InsetInclude.cpp
index 3cf3189..1bd1c5b 100755
--- a/src/insets/InsetInclude.cpp
+++ b/src/insets/InsetInclude.cpp
@@ -848,8 +848,11 @@ void InsetInclude::latex(otexstream & os, OutputParams const & runparams) const
 		Language const * const oldLang = runparams.master_language;
 		// If the master uses non-TeX fonts (XeTeX, LuaTeX),
 		// the children must be encoded in plain utf8!
-		runparams.encoding = masterBuffer->params().useNonTeXFonts ?
-			encodings.fromLyXName("utf8-plain") : oldEnc;
+		if (masterBuffer->params().useNonTeXFonts)
+			runparams.encoding = encodings.fromLyXName("utf8-plain");
+		else if (oldEnc)
+			runparams.encoding = oldEnc;
+		else runparams.encoding = &tmp->params().encoding();
 		runparams.master_language = buffer().params().language;
 		runparams.par_begin = 0;
 		runparams.par_end = tmp->paragraphs().size();