dwarf_loader: Handle unsupported_tag return in die__process_class()

When die__process_tag() gets some tag it hasn't support for, it returns
the special zeroed 'unsupported_tag' struct tag pointer, but
die__process_class() wasn't handling that, assuming since it isn't NULL
that it is a valid 'struct tag' pointer and proceeded to try to use its
->priv area, b00m.

So catch that and print an "unsuported tag FOO" for that case, this
makes the code way more robust as any unsupported tag will not cause a
segfault, just a warning.

So, for the case in the Bugtracker tag below, we don't segfault instead
we show just that DW_TAG_variant_part isn't supported when found inside
a DW_TAG_structure_type (or DW_TAG_class_type) as is the case with ADA
95.

Reported-by: Tom de Vries
Bugtracker: https://github.com/acmel/dwarves/issues/9#issuecomment-697250246
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

dwarf_loader.c

@@ -1350,6 +1350,11 @@ static int die__process_class(Dwarf_Die *die, struct type *class,
 			if (tag == NULL)
 				return -ENOMEM;
 
+			if (tag == &unsupported_tag) {
+				tag__print_not_supported(dwarf_tag(die));
+				continue;
+			}
+
 			uint32_t id;
 
 			if (cu__table_add_tag(cu, tag, &id) < 0) {