Fix self-aliasing in json_pointer::operator/= and patch_inplace

json_pointer::operator/=(const json_pointer&) passed iterators into its
own token vector to insert() when a pointer was appended to itself
(p /= p), which is undefined behavior; libstdc++ produced "/a/b//" for
"/a/b". The pointer now appends a copy in that case.

basic_json::patch_inplace iterated the patch while changing *this, so
j.patch_inplace(j) read freed memory. It now applies a copy of the
patch when the patch is the value itself. A patch stored inside the
value remains undefined behavior and is documented as such; copying the
patch unconditionally would double the time of a large patch.

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

include/nlohmann/detail/json_pointer.hpp

@@ -243,6 +243,13 @@ class json_pointer
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_slasheq/
     json_pointer& operator/=(const json_pointer& ptr)
     {
+        // insert() must not be given iterators into the vector it inserts
+        // into: append a copy when a pointer is appended to itself
+        if (JSON_HEDLEY_UNLIKELY(&ptr == this))
+        {
+            return *this /= json_pointer(ptr);
+        }
+
         reference_tokens.insert(reference_tokens.end(),
                                 ptr.reference_tokens.begin(),
                                 ptr.reference_tokens.end());