Fix double lookup in `to<JsonVariant>()` #739

Job	Run time
Lint	30s
GCC (5)	3m 25s
GCC (6)	3m 52s
GCC (7, -fsanitize=leak -fno-sanitize-recover=all)	3m 56s
GCC (8, -fsanitize=undefined -fno-sanitize-recover=all)	4m 32s
GCC (9, -fsanitize=address -fno-sanitize-recover=all)	4m 5s
GCC (10, -funsigned-char)	3m 51s
GCC (11)	3m 43s
Clang (3.8, -stdlib=libc++)	4m 2s
Clang (3.9, -stdlib=libc++)	4m 47s
Clang (4.0, -stdlib=libc++)	3m 23s
Clang (5.0)	3m 48s
Clang (6.0)	4m 15s
Clang (7)	3m 45s
Clang (8, -fsanitize=leak -fno-sanitize-recover=all)	3m 56s
Clang (9, -fsanitize=undefined -fno-sanitize-recover=all)	3m 44s
Clang (10, -fsanitize=address -fno-sanitize-recover=all)	4m 36s
Particle (argon)	29s
GCC for ARM processor	3m 43s
Coverage	2m 34s
Arduino (arduino:avr, arduino:avr:uno)	40s
Arduino (arduino:samd, arduino:samd:mkr1000)	1m 16s
PlatformIO (atmelavr, leonardo, SD, Ethernet, avr)	1m 0s
PlatformIO (espressif8266, huzzah, esp8266)	3m 56s
PlatformIO (espressif32, esp32dev, Ethernet, esp8266)	3m 22s
PlatformIO (atmelsam, mkr1000USB, SD, Ethernet, esp8266)	1m 42s
PlatformIO (teensy, teensy31, esp8266)	1m 51s
PlatformIO (ststm32, adafruit_feather_f405, SD, Ethernet, esp8266)	3m 30s
PlatformIO (nordicnrf52, adafruit_feather_nrf52840, SD, Ethernet, esp8266)	2m 31s
Amalgamate ArduinoJson.h	6s
Amalgamate ArduinoJson.hpp	5s
CodeQL	12m 10s
ESP-IDF	4m 0s
Valgrind	3m 10s
Test configuration on Linux	41s
Test configuration on Windows	55s
Clang-Tidy	5m 26s
XCode (11.7)	4m 9s
XCode (12.4)	5m 14s
XCode (13.2.1)	4m 59s
	2h 11m 39s

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