turn -Werror=null-dereference into a warning

There is unfortunately no way to suppres this witha pragma,
and gcc 9 uncovers quite a few new instaces with enabled LTO that can't/won't be fixed
"error: potential null pointer dereference"

Related: OS#4123
Change-Id: I4d1219bf84d3b8dcaf925a60cf54abe733fba263
1 file changed