[poincare] A non defined symbol is not real

This fixes the equation "conj(x) * x +1", that should not be treated as
a polynom

poincare/src/symbol_abstract.cpp

@@ -74,7 +74,7 @@ Expression SymbolAbstract::Expand(const SymbolAbstract & symbol, Context & conte
 bool SymbolAbstract::isReal(const SymbolAbstract & symbol, Context & context) {
   Expression e = SymbolAbstract::Expand(symbol, context, false);
   if (e.isUninitialized()) {
-    return true;
+    return false;
   }
   return e.isReal(context);
 }