Ignore ASM labels for figuring out transport version constant strings (elastic#135702)

This fixes a bug related to transport version constant 
strings where we can get a label instruction instead of 
an ldc instruction during collection of transport references.

Author: jdconrad

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/CollectTransportVersionReferencesTask.java

@@ -101,6 +101,12 @@ public void visitLineNumber(int line, Label start) {
                         lineNumber = line;
                     }
 
+                    @Override
+                    public void visitLabel(Label label) {
+                        // asm uses many debug labels that we do not want to consider
+                        // so we ignore labels so they do not become part of the instructions list
+                    }
+
                     @Override
                     public void visitMethodInsn(int opcode, String owner, String name, String descriptor, boolean isInterface) {
                         if (owner.equals(TRANSPORT_VERSION_SET_CLASS) && name.equals(TRANSPORT_VERSION_SET_METHOD_NAME)) {