pc: fix leak in pc_system_flash_cleanup_unused

pc: fix leak in pc_system_flash_cleanup_unused

tries to fix a leak detected when building with --enable-sanitizers:
./i386-softmmu/qemu-system-i386
Upon exit:
==13576==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 1216 byte(s) in 1 object(s) allocated from:
    #0 0x7f9d2ed5c628 in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.5)
    #1 0x7f9d2e963500 in g_malloc (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.)
    #2 0x55fa646d25cc in object_new_with_type /tmp/qemu/qom/object.c:686
    #3 0x55fa63dbaa88 in qdev_new /tmp/qemu/hw/core/qdev.c:140
    #4 0x55fa638a533f in pc_pflash_create /tmp/qemu/hw/i386/pc_sysfw.c:88
    #5 0x55fa638a54c4 in pc_system_flash_create /tmp/qemu/hw/i386/pc_sysfw.c:106
    #6 0x55fa646caa1d in object_init_with_type /tmp/qemu/qom/object.c:369
    #7 0x55fa646d20b5 in object_initialize_with_type /tmp/qemu/qom/object.c:511
    #8 0x55fa646d2606 in object_new_with_type /tmp/qemu/qom/object.c:687
    #9 0x55fa639431e9 in qemu_init /tmp/qemu/softmmu/vl.c:3878
    #10 0x55fa6335c1b8 in main /tmp/qemu/softmmu/main.c:48
    #11 0x7f9d2cf06e0a in __libc_start_main ../csu/libc-start.c:308
    #12 0x55fa6335f8e9 in _start (/tmp/qemu/build/i386-softmmu/qemu-system-i386)

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20200701145231.19531-1-alxndr@bu.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>