This is reported by the gcc static analyzer (-fanalyzer); it is just test coding so not very critical but probably should still be adjusted .

/jdk/test/jdk/java/lang/String/nativeEncoding/libstringPlatformChars.c:74:12: warning: leak of 'str' [CWE-401] [-Wanalyzer-malloc-leak]
   74 | return JNU_NewStringPlatform(env, str);