Refine use of substring operations

Closes gh-25445

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

@@ -269,7 +269,7 @@ public class StompDecoder {
 		int index = inString.indexOf('\\');
 
 		while (index >= 0) {
-			sb.append(inString.substring(pos, index));
+			sb.append(inString, pos, index);
 			if (index + 1 >= inString.length()) {
 				throw new StompConversionException("Illegal escape sequence at index " + index + ": " + inString);
 			}

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

@@ -212,7 +212,7 @@ public class StompEncoder  {
 	private StringBuilder getStringBuilder(@Nullable StringBuilder sb, String inString, int i) {
 		if (sb == null) {
 			sb = new StringBuilder(inString.length());
-			sb.append(inString.substring(0, i));
+			sb.append(inString, 0, i);
 		}
 		return sb;
 	}