Avoid allocations from StompDecoder#unescape

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompDecoder.java

@@ -264,9 +264,12 @@ private void readHeaders(ByteBuffer byteBuffer, StompHeaderAccessor headerAccess
 	 * <a href="https://stomp.github.io/stomp-specification-1.2.html#Value_Encoding">"Value Encoding"</a>.
 	 */
 	private String unescape(String inString) {
-		StringBuilder sb = new StringBuilder(inString.length());
-		int pos = 0;  // position in the old string
 		int index = inString.indexOf('\\');
+		if (index == -1) {
+			return inString;
+		}
+		int pos = 0;  // position in the old string
+		StringBuilder sb = new StringBuilder(inString.length());
 
 		while (index >= 0) {
 			sb.append(inString, pos, index);