gcc -Wmissing-field-initializers noticed several struct initializations

gcc -Wmissing-field-initializers noticed several struct initializations
that didn't explicitly list all fields.  Most were of no consequence because
they were for static data, so they were all initialized properly.  However,
the two in mhlsbr.c were missing an initialization of other than their last
field, and they contained some subsequent values other than zero.  So, those
later fields were initialized incorrectly.  Also, changed the initialization
of global.c_ovtxt from "" to NULL because free_queue () freed it if non-NULL,
though I think the "" was always overwritten.