Classes that aren't generic but inherit from a generic type have a signature

This is a lazy fix to stop trying to create java_generics_class_typet
when the signature doesn't have the correct format

Author: thk123

src/java_bytecode/java_bytecode_convert_class.cpp

@@ -94,7 +94,7 @@ void java_bytecode_convert_classt::convert(const classt &c)
   }
 
   java_class_typet class_type;
-  if(c.signature.has_value())
+  if(c.signature.has_value() && c.signature.value()[0]=='<')
   {
     java_generics_class_typet generic_class_type;
 #ifdef DEBUG