x86/boot: Handle malformed SRAT tables during early ACPI parsing

Break an infinite loop when early parsing of the SRAT table is caused
by a subtable with zero length. Known to affect the ASUS WS X299 SAGE
motherboard with firmware version 1201 which has a large block of
zeros in its SRAT table. The kernel could boot successfully on this
board/firmware prior to the introduction of early parsing this table or
after a BIOS update.

 [ bp: Fixup whitespace damage and commit message. Make it return 0 to
   denote that there are no immovable regions because who knows what
   else is broken in this BIOS. ]

Fixes: 02a3e3cd

 ("x86/boot: Parse SRAT table and count immovable memory regions")
Signed-off-by: Steven Clarkson <sc@lambdal.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: linux-acpi@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206343
Link: https://lkml.kernel.org/r/CAHKq8taGzj0u1E_i=poHUam60Bko5BpiJ9jn0fAupFUYexvdUQ@mail.gmail.com