Fixes #1425: prevent StrinIndexOutOfBoundsException from JsonPointer.head()

Author: cowtowncoder

src/test/java/com/fasterxml/jackson/core/jsonptr/JsonPointer1425Test.java

@@ -0,0 +1,26 @@
+package com.fasterxml.jackson.core.jsonptr;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.core.JUnit5TestBase;
+import com.fasterxml.jackson.core.JsonPointer;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class JsonPointer1425Test extends JUnit5TestBase
+{
+
+    // [core#1425]
+    @Test
+    public void test1425() {
+        JsonPointer ptr = JsonPointer.compile("/a/b/0/qwerty");
+        JsonPointer head = ptr.head();
+        assertEquals("/a/b/0", head.toString());
+        head = head.head(); // Exception happens here
+        assertEquals("/a/b", head.toString());
+        head = head.head();
+        assertEquals("/a", head.toString());
+        head = head.head();
+        assertEquals("", head.toString());
+    }
+}