staging/easycap: remove explicit NULL initialization

remove intializations to NULL where not needed and let the compiler
find flows with unitilized variables.
Fix one such flow in easycap_vma_fault function

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>