Added an invalid instruction hook (#1132)

* first draft for an invalid instruction hook

* Fixed documentation on return value of invalid insn hook

include/uc_priv.h

@@ -107,6 +107,7 @@ enum uc_hook_idx {
     UC_HOOK_MEM_WRITE_IDX,
     UC_HOOK_MEM_FETCH_IDX,
     UC_HOOK_MEM_READ_AFTER_IDX,
+    UC_HOOK_INSN_INVALID_IDX,
 
     UC_HOOK_MAX,
 };