Fix a stack uninitialized variable bug

[Mem2019]

This bug can cause "callsite_dominators" to generate a key with wrong number, which will cause following error when running fuzzer.

thread 'main' panicked at 'Could not read cfg targets file: Custom { kind: InvalidData, error: Error("invalid digit found in string", line: 5, column: 1) }', fuzzer/src/fuzz_main.rs:42:72