Avoid an infinite loop

When pasting "\big" without any following delimiter, avoid
processing the same token again and again. For unknown reasons,
the delim docstring turns out to always be not empty: even when
it simply contains a '0' (no delimiter follows), its length is 1.

Fixes bug #11027.

(cherry picked from commit 8e8b3f6104)

src/mathed/MathParser.cpp

@@ -2000,7 +2000,10 @@ bool Parser::parse1(InsetMathGrid & grid, unsigned flags,
 							new InsetMathBig(t.cs(), delim)));
 					else {
 						cell->push_back(createInsetMath(t.cs(), buf));
-						putback();
+						// For some reason delim.empty()
+						// is always false here
+						if (delim.at(0))
+							putback();
 					}
 				}