[LyX/master] Fix math class code in InsetMathColor

[Jean-Marc Lasgouttes]

commit c3114515180de263597be65cc9b8724798278b7f
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Mon Feb 8 13:13:21 2021 +0100

    Fix math class code in InsetMathColor
    
    The current code is too naïve and asserts on MC_UNKNOWN class.
    
    See discussion here:
    https://marc.info/?l=lyx-devel&m=161267112524889&w=2
---
 src/mathed/InsetMathColor.cpp |    8 ++------
 src/mathed/MathData.cpp       |   11 +++++++++++
 src/mathed/MathData.h         |    2 ++
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/src/mathed/InsetMathColor.cpp b/src/mathed/InsetMathColor.cpp
index f2f9116..13e6fa4 100644
--- a/src/mathed/InsetMathColor.cpp
+++ b/src/mathed/InsetMathColor.cpp
@@ -98,12 +98,8 @@ void InsetMathColor::write(TeXMathStream & os) const
 {
 	// We have to ensure correct spacing when the front and/or back
 	// atoms are not ordinary ones (bug 11827).
-	docstring const frontclass =
-		!cell(0).empty() ? class_to_string(cell(0).front()->mathClass())
-		               : from_ascii("mathord");
-	docstring const backclass =
-		!cell(0).empty() ? class_to_string(cell(0).back()->mathClass())
-		               : from_ascii("mathord");
+	docstring const frontclass = class_to_string(cell(0).firstMathClass());
+	docstring const backclass = class_to_string(cell(0).lastMathClass());
 	bool adjchk = os.latex() && !os.inMathClass() && (normalcolor(color_) || oldstyle_);
 	bool adjust_front = frontclass != "mathord" && adjchk;
 	bool adjust_back = backclass != "mathord" && adjchk;
diff --git a/src/mathed/MathData.cpp b/src/mathed/MathData.cpp
index c9db36c..d3ce3a3 100644
--- a/src/mathed/MathData.cpp
+++ b/src/mathed/MathData.cpp
@@ -969,6 +969,17 @@ MathClass MathData::mathClass() const
 }
 
 
+MathClass MathData::firstMathClass() const
+{
+	for (MathAtom const & at : *this) {
+		MathClass mc = at->mathClass();
+		if (mc != MC_UNKNOWN)
+			return mc;
+	}
+	return MC_ORD;
+}
+
+
 MathClass MathData::lastMathClass() const
 {
 	MathClass res = MC_ORD;
diff --git a/src/mathed/MathData.h b/src/mathed/MathData.h
index ebc59d4..865742d 100644
--- a/src/mathed/MathData.h
+++ b/src/mathed/MathData.h
@@ -144,6 +144,8 @@ public:
 	void drawT(TextPainter & pi, int x, int y) const;
 	/// approximate mathclass of the data
 	MathClass mathClass() const;
+	/// math class of first interesting element
+	MathClass firstMathClass() const;
 	/// math class of last interesting element
 	MathClass lastMathClass() const;
 	/// is the cell in display style