fuzz: unpoison result of mutate_{byte,int}()

LLVMFuzzerMutate() may return data marked as uninitialized but our value
mutators assume that the entire region is initialized. MSAN recently got
stricter in how it checks use of these potentially uninitialized values.
Manually unpoison the response from LLVMFuzzerMutate() for these two
functions.

fuzz/mutator_aux.c

@@ -135,12 +135,18 @@ void
 mutate_byte(uint8_t *b)
 {
 	LLVMFuzzerMutate(b, sizeof(*b), sizeof(*b));
+#ifdef WITH_MSAN
+	__msan_unpoison(b, sizeof(*b));
+#endif
 }
 
 void
 mutate_int(int *i)
 {
 	LLVMFuzzerMutate((uint8_t *)i, sizeof(*i), sizeof(*i));
+#ifdef WITH_MSAN
+	__msan_unpoison(i, sizeof(*i));
+#endif
 }
 
 void