Adding an early guard for correctly parsed exception table

This must be true, so catching this early is an indicator that something
has gone wrong earlier when parsing this bytecode file.

Author: thk123

src/java_bytecode/java_bytecode_parser.cpp

@@ -947,6 +947,12 @@ void java_bytecode_parsert::rmethod_attribute(methodt &method)
     {
       u2 start_pc=read_u2();
       u2 end_pc=read_u2();
+
+      INVARIANT(
+        start_pc < end_pc,
+        "The start_pc must be less than the end_pc as this is the range the "
+        "exception is active");
+
       u2 handler_pc=read_u2();
       u2 catch_type=read_u2();
       method.exception_table[e].start_pc=start_pc;