ShellPkg: acpiview: Set ItemPtr to NULL for unprocessed table fields

ShellPkg: acpiview: Set ItemPtr to NULL for unprocessed table fields

For fields outside the buffer length provided, reset any pointers,
which were supposed to be updated by a ParseAcpi() function call to
NULL. This way one can easily validate if a pointer was successfully
updated.

The ParseAcpi() function parses the given ACPI table buffer by a
number of bytes which is a minimum of the buffer length and the length
described by ACPI_PARSER array. If the buffer length is shorter than
the array describing how to process the ACPI structure, then it is
possible that the ItemPtr inside ACPI_PARSER may not get updated or
initialized. This can lead to an error if the value pointed to by
ItemPtr is later used to control the parsing logic.

A typical example would be a 'number of elements' field in an ACPI
structure header which defines how many substructures of a given type
are present in the structure body. If the 'number of elements' field
is not parsed, we will have a dangling pointer which could cause a
problem later.

Signed-off-by: Krzysztof Koch <krzysztof.koch@arm.com>