acpi-erst: divide by zero in make_erst_storage_header()
Description of problem:
When we gives `0` to `record_size` for `acpi-erst` device, below code may triggers divide-by-zero.

```c
static void make_erst_storage_header(ERSTDeviceState *s)
    ...
    header->magic = cpu_to_le64(ERST_STORE_MAGIC);
    header->record_size = cpu_to_le32(s->default_record_size);
    header->version = cpu_to_le16(0x0100);
    header->reserved = cpu_to_le16(0x0000);

    /* Compute mapsize */
    mapsz = s->storage_size / s->default_record_size; // devide-by-zero occurs
```

`acpi-erst` device refuses invalid value for `record_size` and does appropriate condition check in `check_erst_backend_storage()`, but this check is placed before the function triggering the error when `header->magic` is 0.

```c
static void check_erst_backend_storage(ERSTDeviceState *s, Error **errp)
    ...
    /*
     * Check if header is uninitialized; HostMemoryBackend inits to 0
     */
    if (le64_to_cpu(header->magic) == 0UL) {
        make_erst_storage_header(s);
    }

    /* Validity check record_size */
    record_size = le32_to_cpu(header->record_size);
    if (!(
        (record_size) && /* non zero */
        (record_size >= UEFI_CPER_RECORD_MIN_SIZE) &&
        (((record_size - 1) & record_size) == 0) && /* is power of 2 */
        (record_size >= 4096) /* PAGE_SIZE */
        )) {
        error_setg(errp, "ERST record_size %u is invalid", record_size);
        return;
    }
```
Steps to reproduce:
1. make sure `acpi-erst.backing` doesn't exist in current folder.
2. run qemu command.
```bash
./build/qemu-system-i386 -object memory-backend-file,id=erstnvram,mem-path=acpi-erst.backing,size=0x10000,share=on -device acpi-erst,memdev=erstnvram,record_size=0
```
Additional information:
I built qemu from source code with `--enable-sanitizers`, and backtrace is as follows:
```bash
AddressSanitizer:DEADLYSIGNAL
=================================================================
==401519==ERROR: AddressSanitizer: FPE on unknown address 0x55bd0616fd53 (pc 0x55bd0616fd53 bp 0x61f000000e80 sp 0x7fffd16e5d90 T0)
    #0 0x55bd0616fd53 in make_erst_storage_header /home/xxx/qemu/build/../hw/acpi/erst.c:401
    #1 0x55bd0616fd53 in check_erst_backend_storage /home/xxx/qemu/build/../hw/acpi/erst.c:431
    #2 0x55bd0616fd53 in erst_realizefn /home/xxx/qemu/build/../hw/acpi/erst.c:973
    #3 0x55bd06268426 in pci_qdev_realize /home/xxx/qemu/build/../hw/pci/pci.c:2093
    #4 0x55bd06557629 in device_set_realized /home/xxx/qemu/build/../hw/core/qdev.c:510
    #5 0x55bd0655ecc8 in property_set_bool /home/xxx/qemu/build/../qom/object.c:2362
    #6 0x55bd0655cec4 in object_property_set /home/xxx/qemu/build/../qom/object.c:1471
    #7 0x55bd06560dec in object_property_set_qobject /home/xxx/qemu/build/../qom/qom-qobject.c:28
    #8 0x55bd0655d30a in object_property_set_bool /home/xxx/qemu/build/../qom/object.c:1541
    #9 0x55bd0632f8cf in qdev_device_add_from_qdict /home/xxx/qemu/build/../system/qdev-monitor.c:719
    #10 0x55bd0632fc91 in qdev_device_add /home/xxx/qemu/build/../system/qdev-monitor.c:738
    #11 0x55bd0633ae7e in device_init_func /home/xxx/qemu/build/../system/vl.c:1203
    #12 0x55bd066e7a50 in qemu_opts_foreach /home/xxx/qemu/build/../util/qemu-option.c:1135
    #13 0x55bd06335421 in qemu_create_cli_devices /home/xxx/qemu/build/../system/vl.c:2640
    #14 0x55bd06335421 in qmp_x_exit_preconfig /home/xxx/qemu/build/../system/vl.c:2709
    #15 0x55bd06338f42 in qemu_init /home/xxx/qemu/build/../system/vl.c:3742
    #16 0x55bd06553e35 in main /home/xxx/qemu/build/../system/main.c:47
    #17 0x7efcdb919d8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #18 0x7efcdb919e3f in __libc_start_main_impl ../csu/libc-start.c:392
    #19 0x55bd060ecb24 in _start (/home/xxx/qemu/build/qemu-system-i386+0x32db24)
```