Minor memory leak reported for Metal Rendering pipeline

This came up as a review comment on https://github.com/openjdk/jdk/pull/2403.

Please check the validity and fix if it's a valid leak.

According to Xcode Instruments leak profile, there are 2 minor memory leaks in the Metal rendering pipeline:

#1 Malloc 80 Bytes 1 0x7fde0d4247b0 80 Bytes libjava.dylib getStringUTF8

   0 libsystem_malloc.dylib malloc_zone_malloc
   1 libsystem_malloc.dylib malloc
   2 libjava.dylib getStringUTF8 /Volumes/Work/review/2403/jdk/src/java.base/share/native/libjava/jni_util.c:888
   3 libjava.dylib JNU_GetStringPlatformChars /Volumes/Work/review/2403/jdk/src/java.base/share/native/libjava/jni_util.c:917
   4 libawt_lwawt.dylib Java_sun_java2d_metal_MTLGraphicsConfig_getMTLConfigInfo /Volumes/Work/review/2403/jdk/src/java.desktop/macosx/native/libawt_lwawt/java2d/metal/MTLGraphicsConfig.m:151
   5 0x11ab08d48
   6 0x11ab0250d

#2 Malloc 80 Bytes 1 0x7fde0d4325a0 80 Bytes libjava.dylib getStringUTF8

   0 libsystem_malloc.dylib malloc_zone_malloc
   1 libsystem_malloc.dylib malloc
   2 libjava.dylib getStringUTF8 /Volumes/Work/review/2403/jdk/src/java.base/share/native/libjava/jni_util.c:888
   3 libjava.dylib JNU_GetStringPlatformChars /Volumes/Work/review/2403/jdk/src/java.base/share/native/libjava/jni_util.c:917
   4 libawt_lwawt.dylib Java_sun_java2d_metal_MTLGraphicsConfig_tryLoadMetalLibrary /Volumes/Work/review/2403/jdk/src/java.desktop/macosx/native/libawt_lwawt/java2d/metal/MTLGraphicsConfig.m:120
   5 0x11ab08d48
   6 0x11ab024c8