Defensive String equals comparison in ClassReader

Issue: SPR-12382

spring-core/src/main/java/org/springframework/asm/ClassReader.java

@@ -928,7 +928,8 @@ public class ClassReader {
          */
         if (WRITER && mv instanceof MethodWriter) {
             MethodWriter mw = (MethodWriter) mv;
-            if (mw.cw.cr == this && signature == mw.signature) {
+            if (mw.cw.cr == this &&
+					(signature != null ? signature.equals(mw.signature) : mw.signature == null)) {
                 boolean sameExceptions = false;
                 if (exceptions == null) {
                     sameExceptions = mw.exceptionCount == 0;