check for valid BLOCK/ENDBLK structure
check for valid BLOCK/ENDBLK structure

Content of a block is ignored if the BLOCK or ENDBLK
entity is missing:

- BLOCK ...ignored content... BLOCK ... ENDBLK
- BLOCK ... ENDBLK ...ignored content... ENDBLK

mozman/ezdxf,mozman/ezdxf,mozman/ezdxf,mozman/ezdxf,mozman/ezdxf