FTBFS with gcc-6

[Sean Whitton]

Hello,

geis fails to build with gcc-6:

    geis_subscription.c: In function ‘geis_subscription_bag_empty’:
    geis_subscription.c:240:5: error: this ‘if’ clause does not guard... [-Werror=misleading-indentation]
         if (bag->sub_store[i])
         ^~
    geis_subscription.c:242:7: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the ‘if’
           bag->sub_store[i] = NULL;
           ^~~

I have attached a patch to fix this.