Fix buffer overflows, null pointer dereferences, and unsafe string ops in libdmg

[Copilot]

Several memory safety bugs in the DMG parsing and writing paths that could cause crashes on malformed input or buffer overflows on unbounded string operations.

Changes

Buffer overflow (abstractfile.c)

• vsprintf into a fixed 1024-byte stack buffer → vsnprintf with bounds

NULL pointer dereferences

All cases follow the same pattern: getResourceByKey(...) can return NULL but the result was immediately dereferenced without a check.

• dmgfile.c — openDmgFile: Gracefully return NULL (with resource cleanup) if "blkx" key is absent
• resources.c — readNSiz: Return NULL if "nsiz" key is absent
• resources.c — getDataByID: Added NULL guard on the resource parameter itself
• partition.c — readApplePartitionMap: Chained getResourceByKey + getDataByID with no checks on either
• dmglib.c: Three functions (extractDmg, calculateMasterChecksum, convertToISO) all directly dereferenced getResourceByKey results

Unsafe strcpy of caller-supplied string (partition.c)

createApplePartitionMap used strcpy to write volumeType into a fixed 32-byte pmParType field:

// Before — unchecked copy of caller-supplied string
strcpy((char*)partition[0].pmParType, volumeType);

// After — bounded copy with explicit null termination
strncpy((char*)partition[0].pmParType, volumeType, sizeof(partition[0].pmParType) - 1);
partition[0].pmParType[sizeof(partition[0].pmParType) - 1] = '\0';

Debug information leak (partition.c)

Removed a leftover printf in writeATAPI that printed the internal malloc address and allocation size of a buffer.

Original prompt

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