[lldb] Don't crash on malformed filesets (llvm#98388)

The memory read can fail for a malformed fileset. Handle it gracefully
instead of passing a nullptr to the std::string constructor.

rdar://131477833
(cherry picked from commit 919caa8)

Author: JDevlieghere

lldb/source/Plugins/ObjectContainer/Mach-O-Fileset/ObjectContainerMachOFileset.cpp

@@ -159,9 +159,9 @@ ParseFileset(DataExtractor &data, mach_header header,
       fileset_entry_command entry;
       data.CopyData(load_cmd_offset, sizeof(fileset_entry_command), &entry);
       lldb::offset_t entry_id_offset = load_cmd_offset + entry.entry_id;
-      const char *id = data.GetCStr(&entry_id_offset);
-      entries.emplace_back(entry.vmaddr + slide, entry.fileoff,
-                           std::string(id));
+      if (const char *id = data.GetCStr(&entry_id_offset))
+        entries.emplace_back(entry.vmaddr + slide, entry.fileoff,
+                             std::string(id));
     }
 
     offset = load_cmd_offset + lc.cmdsize;