Fix undefined behavior when disassembling ud0 x86 instructions (#1032)

* fix undefined behavior due to uninitialized memory

* fix bad calloc call
diff --git a/MCInst.c b/MCInst.c
index d521845..cc062d0 100644
--- a/MCInst.c
+++ b/MCInst.c
@@ -16,6 +16,7 @@
 
 void MCInst_Init(MCInst *inst)
 {
+	inst->Opcode = 0;
 	inst->OpcodePub = 0;
 	inst->size = 0;
 	inst->has_imm = false;