Guard the null value of a value-initialized iterator; drop the unneeded found flag

With the root's byte range, diagnostics() reaches get_byte_positions() also
with JSON_DIAGNOSTICS when there is no value, as for a value-initialized
iterator (invalid_iterator.212). get_byte_positions() now returns no range
for a null value, the same change as #6003, so either can be merged first.

In the view, a value that value_path() does not find leaves no tokens, so the
found flag added nothing to the tokens.empty() test.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/exceptions.hpp

@@ -223,7 +223,8 @@ class exception : public std::exception
     template<typename BasicJsonType>
     static std::string get_byte_positions(const BasicJsonType* leaf_element)
     {
-        if ((leaf_element->start_pos() != std::string::npos) && (leaf_element->end_pos() != std::string::npos))
+        // a value-initialized iterator has no value (invalid_iterator.212)
+        if (leaf_element != nullptr && leaf_element->start_pos() != std::string::npos && leaf_element->end_pos() != std::string::npos)
         {
             return concat("(bytes ", std::to_string(leaf_element->start_pos()), "-", std::to_string(leaf_element->end_pos()), ") ");
         }

include/nlohmann/detail/view/errors.hpp

@@ -255,8 +255,9 @@ std::string diagnostics(const error_site site)
     std::string context;
 #if JSON_DIAGNOSTICS
     std::vector<std::string> tokens;
-    const bool found = edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens);
-    if (found && !tokens.empty())
+    // a value that is not found leaves no tokens, like the root
+    static_cast<void>(edited ? value_path<true>(d, site.value(), tokens) : value_path<false>(d, site.value(), tokens));
+    if (!tokens.empty())
     {
         // the root value has no path, but still has a byte range
         std::string path;