fixing segfault on freeObjectTable

When the object table is empty, freeObjectTable tried to free a
table of size 1, instead of doing nothing. This is now fixed.
This case could happen with corrupted emf files.
options to disable checks on test script and update README

tests/resources/check_correctness.sh:
* option -x to disable svg correctness check
* option -X to ignore emf2svg error codes

(useful for testing emf2svg against corrupted files)

README.md:
* update the command to checks against corrupted files
* cosmetic changes