Make json_pointer::back const (resolves #1764)

include/nlohmann/detail/json_pointer.hpp

@@ -267,7 +267,7 @@ class json_pointer
 
     @since version 3.6.0
     */
-    const std::string& back()
+    const std::string& back() const
     {
         if (JSON_HEDLEY_UNLIKELY(empty()))
         {

single_include/nlohmann/json.hpp

@@ -10232,7 +10232,7 @@ class json_pointer
 
     @since version 3.6.0
     */
-    const std::string& back()
+    const std::string& back() const
     {
         if (JSON_HEDLEY_UNLIKELY(empty()))
         {

test/src/unit-json_pointer.cpp

@@ -559,6 +559,10 @@ TEST_CASE("JSON pointers")
         CHECK(!ptr.empty());
         CHECK(j[ptr] == j["answer"]["everything"]);
 
+        // check access via const pointer
+        const auto cptr = ptr;
+        CHECK(cptr.back() == "everything");
+
         ptr.pop_back();
         ptr.pop_back();
         CHECK(ptr.empty());