OvmfPkg/AcpiPlatformDxe: suppress invalid "deref of undef pointer" warning

OvmfPkg/AcpiPlatformDxe: suppress invalid "deref of undef pointer" warning

RH covscan emits the following false positive:

> Error: CLANG_WARNING:
> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/AcpiPlatform.c:182:14:
> warning: Dereference of undefined pointer value
> #    Status = FwVol->ReadSection (
> #             ^~~~~~~~~~~~~~~~~~
> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/AcpiPlatform.c:164:7: note:
> Assuming the condition is false
> #  if (QemuDetected ()) {
> #      ^~~~~~~~~~~~~~~
> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/AcpiPlatform.c:164:3: note:
> Taking false branch
> #  if (QemuDetected ()) {
> #  ^
> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/AcpiPlatform.c:174:3: note:
> Taking false branch
> #  if (EFI_ERROR (Status)) {
> #  ^
> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/AcpiPlatform.c:180:10: note:
> Assuming 'Status' is equal to EFI_SUCCESS
> #  while (Status == EFI_SUCCESS) {
> #         ^~~~~~~~~~~~~~~~~~~~~
> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/AcpiPlatform.c:180:3: note:
> Loop condition is true.  Entering loop body
> #  while (Status == EFI_SUCCESS) {
> #  ^
> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/AcpiPlatform.c:182:14: note:
> Dereference of undefined pointer value
> #    Status = FwVol->ReadSection (
> #             ^~~~~~~~~~~~~~~~~~
> #  180|     while (Status == EFI_SUCCESS) {
> #  181|
> #  182|->     Status = FwVol->ReadSection (
> #  183|                         FwVol,
> #  184|                         (EFI_GUID*)PcdGetPtr (PcdAcpiTableStorageFile),

This is invalid because LocateFvInstanceWithTables() sets FwVol on
success.

Suppress the message by:
- assigning FwVol NULL first (this would replace the original report with
  "nullptr deref"),
- asserting that FwVol is no longer NULL, on success.

What's important here is that ASSERT() ends with ANALYZER_UNREACHABLE() on
failure.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1710
Issue: scan-0991.txt
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>