iwlwifi: mvm: fix accessing Null pointer during fw dump collection

The firwmare file can come with data that is relevant for paging. This
data is availablet to the firmware upon request, but it stored in the
host's memory. During the firmware init flow, the driver configures the
firmware so that the firwmare knows where is the data.
When paging is used, the variable paging_mem_size is the number of bytes
that are available through paging. This variable is not zeror-ed if the
driver fails to configure the paging in the firmware, but the memory is
freed which is inconsistent.
This inconsistency led to a NULL pointer dereference in the code that
collects the debug data.

Fix this by zero-ing the paging_mem_size variable and NULLify the
relevant pointers, so that the code that collects the debug data will
know that the paging data is not available.

Signed-off-by: Matti Gottlieb <matti.gottlieb@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>