[poincare/unit] Fix temperature conversion

The program would crash when trying to evaluate a unit conversion of the
form "x→_U" where U was a unit of temperature and x did not have any
unit.

poincare/src/unit.cpp

@@ -834,7 +834,7 @@ Expression Unit::ConvertTemperatureUnits(Expression e, Unit unit, ExpressionNode
 
   Expression startUnit;
   e = e.removeUnit(&startUnit);
-  if (startUnit.type() != ExpressionNode::Type::Unit) {
+  if (startUnit.isUninitialized() || startUnit.type() != ExpressionNode::Type::Unit) {
     return Undefined::Builder();
   }
   const Representative * startRepr = static_cast<Unit &>(startUnit).representative();

poincare/test/simplification.cpp

@@ -1267,6 +1267,7 @@ QUIZ_CASE(poincare_simplification_unit_convert) {
   assert_parsed_expression_simplify_to("1→3_m", Undefined::Name());
   assert_parsed_expression_simplify_to("4→_km/_m", Undefined::Name());
   assert_parsed_expression_simplify_to("3×_min→_s+1-1", Undefined::Name());
+  assert_parsed_expression_simplify_to("0→_K", Undefined::Name());
 
   assert_parsed_expression_simplify_to("0_K→_°C", "-273.15×_°C");
   assert_parsed_expression_simplify_to("0_°C→_K", "273.15×_K");