Fix JSON_DIAGNOSTICS test crash and clang-tidy hit in patch_inplace self-aliasing

The spare-capacity test reallocated non-empty elements through get_ref(), which leaves stale parent pointers under JSON_DIAGNOSTICS; reserve while the array is empty instead. Suppress performance-unnecessary-copy-initialization for the copy that is needed because *this is modified.

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

single_include/nlohmann/json.hpp

@@ -43256,6 +43256,7 @@ public:
         // a copy when a value is applied as patch to itself
         if (JSON_HEDLEY_UNLIKELY(&json_patch == this))
         {
+            // NOLINTNEXTLINE(performance-unnecessary-copy-initialization): *this is modified below
             const basic_json json_patch_copy = json_patch;
             patch_inplace(json_patch_copy);
             return;