Fix msan issue

Fix msan issue

The fact that HB_AUTO_RETURN will return rvalue-references for rvalues
is very disturbing.

Even apart from that, I'm totally lost re any hb_move needs or
hb_forward'ing to functions/templates where the type is fixed by
explicitly specifying template parameters.

==1==ERROR: AddressSanitizer: stack-use-after-return on address 0x7f6ad65e51e0 at pc 0x0000005da240 bp 0x7ffc104ab670 sp 0x7ffc104ab668
READ of size 4 at 0x7f6ad65e51e0 thread T0
SCARINESS: 55 (4-byte-read-stack-use-after-return)
     #0 0x5da23f in bool OT::Coverage::serialize<hb_map_iter_t<hb_map_iter_t<hb_filter_iter_t<OT::Coverage::iter_t, hb_set_t const&, $_7&, (void*)0>, OT::SingleSubstFormat1::subset(hb_subset_context_t*) const::'lambda'(unsigned int), (hb_function_sortedness_t)1, (void*)0>, $_20&, (hb_function_sortedness_t)1, (void*)0>, (void*)0>(hb_serialize_context_t*, hb_map_iter_t<hb_map_iter_t<hb_filter_iter_t<OT::Coverage::iter_t, hb_set_t const&, $_7&, (void*)0>, OT::SingleSubstFormat1::subset(hb_subset_context_t*) const::'lambda'(unsigned int), (hb_function_sortedness_t)1, (void*)0>, $_20&, (hb_function_sortedness_t)1, (void*)0>) harfbuzz/src/hb-ot-layout-common.hh:1055:16
     #1 0x5d88f9 in bool OT::SingleSubstFormat1::serialize<hb_map_iter_t<hb_map_iter_t<hb_filter_iter_t<OT::Coverage::iter_t, hb_set_t const&, $_7&, (void*)0>, OT::SingleSubstFormat1::subset(hb_subset_context_t*) const::'lambda'(unsigned int), (hb_function_sortedness_t)1, (void*)0>, $_20&, (hb_function_sortedness_t)1, (void*)0>, (void*)0>(hb_serialize_context_t*, hb_map_iter_t<hb_map_iter_t<hb_filter_iter_t<OT::Coverage::iter_t, hb_set_t const&, $_7&, (void*)0>, OT::SingleSubstFormat1::subset(hb_subset_context_t*) const::'lambda'(unsigned int), (hb_function_sortedness_t)1, (void*)0>, $_20&, (hb_function_sortedness_t)1, (void*)0>, unsigned int) harfbuzz/src/hb-ot-layout-gsub-table.hh:98:9