libsparse: fix double free after block splitting

Due to https://r.android.com/1310496, sparse_file_write() splits all
blocks larger than 64 MiB.  However, the code that splits file-backed
blocks copies the pointer to the filename without duplicating the
underlying memory, causing a double free in backed_block_destroy()
later.  Fix this by using strdup().  Also, as long as that is being
fixed, also check for failure.

Test: SANITIZE_HOST=address mmm external/e2fsprogs
      mkdir mnt
      mkfs.ext4 img 1G
      sudo mount img mnt
      sudo cp /dev/urandom mnt/file
      sudo umount mnt
      ext2simg img simg

Before this fix it gave:

    ==2216498==ERROR: AddressSanitizer: attempting double-free on 0x602000000090 in thread T0:
        #0 0x55a52454c9a2 in free out/stage2/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/out/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:52:3
        #1 0x7ffa247c82ee in backed_block_destroy(backed_block*) system/core/libsparse/backed_block.cpp:106:5
        #2 0x7ffa247c82ee in backed_block_list_destroy(backed_block_list*) system/core/libsparse/backed_block.cpp:124:7
        #3 0x7ffa247cd055 in sparse_file_destroy system/core/libsparse/sparse.cpp:49:3
        #4 0x55a524587b75 in main external/e2fsprogs/contrib/android/ext2simg.c:239:2

Change-Id: I4607ef5adcf6512645342beaf91aff6033414e54