Java front-end: fix vector index type in populate_live_range_holes

CodeQL rightly complained that there is comparison using a more narrow
type on the left-hand side of a less-than in a loop condition, which may
give rise to a non-terminating loop in case of integer overflow.

While at it, also add checks before unconditionally accessing the first
element.

Author: tautschnig

jbmc/src/java_bytecode/java_local_variable_table.cpp

@@ -513,18 +513,17 @@ static void populate_live_range_holes(
     merge_vars.begin(), merge_vars.end());
   std::sort(sorted_by_startpc.begin(), sorted_by_startpc.end(), lt_startpc);
 
+  PRECONDITION(!sorted_by_startpc.empty());
   maybe_add_hole(
     merge_into,
     expanded_live_range_start,
     sorted_by_startpc[0]->var.start_pc);
-  for(java_bytecode_convert_methodt::method_offsett idx = 0;
-      idx < sorted_by_startpc.size() - 1;
-      ++idx)
+  for(auto it = sorted_by_startpc.begin() + 1; it != sorted_by_startpc.end();
+      ++it)
   {
+    auto &local_var = (*std::prev(it))->var;
     maybe_add_hole(
-      merge_into,
-      sorted_by_startpc[idx]->var.start_pc+sorted_by_startpc[idx]->var.length,
-      sorted_by_startpc[idx+1]->var.start_pc);
+      merge_into, local_var.start_pc + local_var.length, (*it)->var.start_pc);
   }
 }