Fix patch::add creating nonexistent parents

The previous behavior was not in accordance with RFC6902.
Add unit test.

Fixes nlohmann#3134.

include/nlohmann/json.hpp

@@ -4683,7 +4683,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             // get reference to parent of JSON pointer ptr
             const auto last_path = ptr.back();
             ptr.pop_back();
-            basic_json& parent = result[ptr];
+            // parent must exist when performing patch add per RFC6902 specs
+            basic_json& parent = result.at(ptr);
 
             switch (parent.m_type)
             {

single_include/nlohmann/json.hpp

@@ -23527,7 +23527,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             // get reference to parent of JSON pointer ptr
             const auto last_path = ptr.back();
             ptr.pop_back();
-            basic_json& parent = result[ptr];
+            // parent must exist when performing patch add per RFC6902 specs
+            basic_json& parent = result.at(ptr);
 
             switch (parent.m_type)
             {

tests/src/unit-json_patch.cpp

@@ -60,6 +60,16 @@ TEST_CASE("JSON patch")
 
             // because "a" does not exist.
             CHECK_THROWS_WITH_AS(doc2.patch(patch), "[json.exception.out_of_range.403] key 'a' not found", json::out_of_range&);
+
+            json doc3 = R"({ "a": {} })"_json;
+            json patch2 = R"([{ "op": "add", "path": "/a/b/c", "value": 1 }])"_json;
+
+            // should cause an error because "b" does not exist in doc3
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(doc3.patch(patch2), "[json.exception.out_of_range.403] (/a) key 'b' not found", json::out_of_range&);
+#else
+            CHECK_THROWS_WITH_AS(doc3.patch(patch2), "[json.exception.out_of_range.403] key 'b' not found", json::out_of_range&);
+#endif
         }
 
         SECTION("4.2 remove")