afl fuzzer setup

Add crash case crash-26-x64-indexreg-already-set.c

Add crash case crash-25-x64-expected-an-immediate-or-register.c

[gardening] Fix typo: "and immediate" → "an immediate"

handle equal directive. this fixes c-crashes/crash-12-x64-cannot-set-a-variable-that-has-already-been-used.c

handle error in applyFixup(). this fixes c-crashes/crash-23-x64-value-does-not-fit-in-the-fixup-field.c