Test for undo crash in math env.

According to fix #9432, Scott found a new crash
	1. start a new LyX document, english GUI
	2. alt+m f to create a fraction
	3. alt+m r to insert a root
	4. undo

development/autotests/bug-math-undo-in.txt

@@ -0,0 +1,10 @@
+# Test undo of \sqrt in \frac
+# 
+
+Lang C
+TestBegin test.lyx > lyx-log.txt 2>&1
+KK: \Amm\\frac \\sqrt \Cz
+KK: \[Return]
+TestEnd
+Lang C
+Assert ! pcregrep -M 'SIGSEGV' lyx-log.txt