Keep diagnostic key paths null-terminated

Building the token from data() and size() kept an embedded null byte in the
key, and since what() hands out a C string, that truncated the whole message
rather than just the key: to_bson() on a key containing U+0000 reported
"[json.exception.out_of_range.409] (/en" instead of the full explanation.
This broke test-bson under JSON_DIAGNOSTICS.

Constructing from data() alone stops at the first null byte, which is what
c_str() did before, so the message is unchanged -- without requiring
string_t to provide c_str().

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

single_include/nlohmann/json.hpp

@@ -4993,7 +4993,10 @@ class exception : public std::exception
                     {
                         if (&element.second == current)
                         {
-                            tokens.emplace_back(element.first.data(), element.first.size());
+                            // data() is null-terminated, so a key containing
+                            // a null byte is cut short here rather than
+                            // truncating the whole message at what()
+                            tokens.emplace_back(element.first.data());
                             break;
                         }
                     }