cxxrtl: fix vcd writer scope handling

The vcd writer incorrectly treated two scope vectors as the same, whenever
they have the same length of entries and the last item matches.
This is however not always true, for example consider a current_scope of
["top", "something0", "same"]
and a scope of
["top", "something1", "same"]

backends/cxxrtl/runtime/cxxrtl/cxxrtl_vcd.h

@@ -50,9 +50,13 @@ class vcd_writer {
 
 	void emit_scope(const std::vector<std::string> &scope) {
 		assert(!streaming);
-		while (current_scope.size() > scope.size() ||
-		       (current_scope.size() > 0 &&
-			current_scope[current_scope.size() - 1] != scope[current_scope.size() - 1])) {
+		size_t same_scope_count = 0;
+		while ((same_scope_count < current_scope.size()) &&
+			   (same_scope_count < scope.size()) &&
+			   (current_scope[same_scope_count] == scope[same_scope_count])) {
+			same_scope_count++;
+		}
+		while (current_scope.size() > same_scope_count) {
 			buffer += "$upscope $end\n";
 			current_scope.pop_back();
 		}