ShellPkg DmpStore: Make NameSize to be consistent with name buffer

ShellPkg DmpStore: Make NameSize to be consistent with name buffer

Current code will allocate pool to hold the null char for name buffer
when PrevName==NULL, but the NameSize is still 0.

For this case, GetNextVariableName will return EFI_INVALID_PARAMETER
to follow UEFI 2.7 spec.

UEFI 2.7 spec:
  The VariableNameSize must not be smaller the size of the variable
  name string passed to GetNextVariableName() on input in the
  VariableName buffer.

  EFI_INVALID_PARAMETER
  Null-terminator is not found in the first VariableNameSize bytes of
  the input VariableName buffer.

This patch is to make NameSize to be consistent with name buffer.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>