OvmfPkg/X86QemuLoadImageLib: add dummy assignment to work around GCC

OvmfPkg/X86QemuLoadImageLib: add dummy assignment to work around GCC

GCC 4.8 or 4.9 may throw the following error when building OVMF:

  Edk2/OvmfPkg/Library/X86QemuLoadImageLib/X86QemuLoadImageLib.c:
      In function ‘QemuLoadKernelImage’:
  Edk2/OvmfPkg/Library/X86QemuLoadImageLib/X86QemuLoadImageLib.c:416:30:
      error: ‘CommandLine’ may be used uninitialized in this function
                                               [-Werror=maybe-uninitialized]
        UnicodeSPrintAsciiFormat (
        cc1: all warnings being treated as errors

This is due to the fact that older GCCs fail to infer that CommandLine is
never actually used unless it has been assigned. So add a redundant NULL
assignment to help these older GCCs understand this.

Link: https://bugzilla.tianocore.org/show_bug.cgi?id=2630
Fixes: 7c47d89003a6f ("OvmfPkg: implement QEMU loader library for X86 with ...")
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>