Fix __slvg_checkpoint() metadata memory leak

Direct leak of 1015 byte(s) in 1 object(s) allocated from:

    #0 0x4dac00 in malloc (/mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/test/salvage/t+0x4dac00)

    #1 0x5b7fdc in __wt_malloc /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/os_common/os_alloc.c:85:11

    #2 0x5b7fdc in __wt_strndup /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/os_common/os_alloc.c:287

    #3 0x5a6293 in __wt_strdup /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/include/misc.i:82:10

    #4 0x5a6293 in __wt_metadata_search /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/meta/meta_table.c:326

    #5 0x77ff8f in __slvg_checkpoint /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/btree/bt_slvg.c:195:2

    #6 0x77ff8f in __wt_salvage /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/btree/bt_slvg.c:396

    #7 0x60f890 in __wt_exclusive_handle_operation /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/schema/schema_worker.c:35:2

    #8 0x60fcef in __wt_schema_worker /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/schema/schema_worker.c:75:4

    #9 0x62ab3c in __session_salvage /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/../src/session/session_api.c:1331:2

    #10 0x515dc0 in process /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/test/salvage/../../../test/salvage/salvage.c:649:2

    #11 0x5145f7 in run /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/test/salvage/../../../test/salvage/salvage.c:437:2

    #12 0x5132fb in main /mnt/data0/jenkins/workspace/wiredtiger-clang-sanitizer/build_posix/test/salvage/../../../test/salvage/salvage.c:101:2

    #13 0x7fa11875ab96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310