[multipathd] fix a segv in enable_group()
[multipathd] fix a segv in enable_group()

pp->mpp->pg should not be NULL when entering enable_group().
Failing update_multipath_strings() can  lead to such a situation.
So check its return value and abord early.

vijaychauhan/multipath-tools,gebi/multipath-tools,vijaychauhan/multipath-tools,grzn/multipath-tools-explained,grzn/multipath-tools-explained,unakatsuo/multipath-tools,unakatsuo/multipath-tools,unakatsuo/multipath-tools