Fix potential use-after-free in JSString

[kateinoigakukun]

The guts' lifetime was not guaranteed to be longer than swjs_value_equals call, which could lead to a use-after-free.

[github-actions]

Time Change: +14ms (0%)

Total Time: 9,580ms

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import with int	22ms	+2ms (9%)	🔍

View Unchanged

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import	24ms	+1ms
Serialization/JavaScript function call from Swift	121ms	+2ms (1%)
Serialization/Swift Int to JavaScript with assignment	329ms	-4ms (1%)
Serialization/Swift Int to JavaScript with call	962ms	-0ms
Serialization/JavaScript Number to Swift Int	267ms	+7ms (2%)
Serialization/Swift String to JavaScript with assignment	410ms	-1ms (0%)
Serialization/Swift String to JavaScript with call	1,051ms	+2ms (0%)
Serialization/JavaScript String to Swift String	3,557ms	+1ms (0%)
Object heap/Increment and decrement RC	2,804ms	+7ms (0%)

View Baselines

Test name	Duration
Serialization/Call JavaScript function directly	8ms
Serialization/Assign JavaScript number directly	7ms
Serialization/Call with JavaScript number directly	6ms
Serialization/Write JavaScript string directly	6ms
Serialization/Call with JavaScript string directly	6ms