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.

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();
 					}
 				}