EFI: map allocation size must be set to zero

Commit 8a753b3f1c ("efi: fix allocation problems if ExitBootServices()
fails") replaced the use of a static (and hence zero-initialized)
variable by an automatic (and hence uninitialized) one.

Also drop the variable introduced by that commit in favor of re-using
another available and suitable one.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -704,7 +704,7 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
     EFI_STATUS status;
     unsigned int i, argc;
     CHAR16 **argv, *file_name, *cfg_file_name = NULL, *options = NULL;
-    UINTN map_alloc_size, map_key, info_size, gop_mode = ~0;
+    UINTN map_key, info_size, gop_mode = ~0;
     EFI_HANDLE *handles = NULL;
     EFI_SHIM_LOCK_PROTOCOL *shim_lock;
     EFI_GRAPHICS_OUTPUT_PROTOCOL *gop = NULL;
@@ -1061,16 +1061,17 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
             efi_arch_video_init(gop, info_size, mode_info);
     }
 
-    efi_bs->GetMemoryMap(&map_alloc_size, NULL, &map_key,
+    info_size = 0;
+    efi_bs->GetMemoryMap(&info_size, NULL, &map_key,
                          &efi_mdesc_size, &mdesc_ver);
-    map_alloc_size += 8 * efi_mdesc_size;
-    efi_memmap = efi_arch_allocate_mmap_buffer(map_alloc_size);
+    info_size += 8 * efi_mdesc_size;
+    efi_memmap = efi_arch_allocate_mmap_buffer(info_size);
     if ( !efi_memmap )
         blexit(L"Unable to allocate memory for EFI memory map");
 
     for ( retry = 0; ; retry = 1 )
     {
-        efi_memmap_size = map_alloc_size;
+        efi_memmap_size = info_size;
         status = efi_bs->GetMemoryMap(&efi_memmap_size, efi_memmap, &map_key,
                                       &efi_mdesc_size, &mdesc_ver);
         if ( EFI_ERROR(status) )