Fix obvious mistakes like spelling errors

src/main/java/com/alibaba/druid/util/StringUtils.java

@@ -51,7 +51,11 @@ public class StringUtils {
         if (indexFrom < 0 || indexTo < 0 || indexFrom > indexTo) {
             return null;
         }
-        indexFrom += start.length();
+
+        if (null != start) {
+            indexFrom += start.length();
+        }
+
         return src.substring(indexFrom, indexTo);
 
     }