msDBFOpenVirtualFile(): validate field extents against record length - #7571

What does this PR do?

AddressSanitizer, reading the first attribute of a shapefile whose .dbf header declares a field wider than its records:

==ERROR: AddressSanitizer: heap-buffer-overflow ... READ of size 10
    #1 msDBFReadAttribute mapxbase.c:647
    #2 msDBFReadStringAttribute mapxbase.c:715
0x... is located 0 bytes after 10-byte region
    allocated by msDBFOpenVirtualFile mapxbase.c:178

msDBFOpenVirtualFile() takes nRecordLength from the header (bytes 10-11) to size pszCurrentRecord, but each field's panFieldSize/panFieldOffset come from the field descriptors with no check that they fit inside the record. msDBFReadAttribute() later strncpys panFieldSize bytes from pabyRec + panFieldOffset, so a header whose fields do not fit the record reads past the record buffer on any attribute access (WFS GetFeature, joins, attribute-driven labels).

Reject the file at open time when the last field's offset+size exceeds nRecordLength, the same layout check shapelib applies. Valid .dbf files satisfy offset+size == nRecordLength for the last field, so they are unaffected. The added unit test covers both the rejected and the accepted case.

What are related issues/pull requests?

None.

AI tool usage

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Tasklist

• Make sure code is correctly formatted (cf pre-commit configuration)
• Add test case(s)
• Add documentation
• Review
• Adjust for comments
• All CI builds and checks have passed