Fix ISOs with missing Apple partition map entries.

We've seen ISOs in the wild (Fedora 34 Workstation DVD) that
have an Apple partition entry, but are missing the APM headers.
Detect this situation and skip the parsing as needed.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

pycdlib/isohybrid.py

@@ -498,13 +498,17 @@ def parse_primary(self, instr, mac):
         offset = 512
         offset += self.header.parse(instr[offset:])
         if mac:
-            # Now go looking for the APMs
+            # Now go looking for the APMs.  Note that we've seen ISOs in the
+            # wild (Fedora 34 Workstation DVD) with Apple partition entries,
+            # but no APM parts.  If we see all zeros where we expect the
+            # first APM part, we skip the parsing here.
             offset = 2048
-            for i_unused in range(0, APM_PARTS):
-                apm_part = APMPartHeader()
-                apm_part.parse(instr[offset:])
-                self.apm_parts.append(apm_part)
-                offset += 2048
+            if instr[offset:offset + 2] != b'\x00\x00':
+                for i_unused in range(0, APM_PARTS):
+                    apm_part = APMPartHeader()
+                    apm_part.parse(instr[offset:])
+                    self.apm_parts.append(apm_part)
+                    offset += 2048
 
         offset = self.header.partition_entries_lba * 512
         for i_unused in range(0, self.header.num_parts):