Slight refactoring to reduce code duplication.

This also comes with a performance bump due to inlining of reg_read/reg_write
(as they're only called once now) and the unlikely() on CHECK_REG_TYPE.

qemu/unicorn_common.h

@@ -129,14 +129,14 @@ static inline void uc_common_init(struct uc_struct* uc)
         uc->release = release_common;
 }
 
-#define CHECK_REG_TYPE(type) do {    \
-    if (size) {                      \
-        if (*size < sizeof(type)) {  \
-            return UC_ERR_NOMEM;     \
-        }                            \
-        *size = sizeof(type);        \
-    }                                \
-    ret = UC_ERR_OK;                 \
+#define CHECK_REG_TYPE(type) do {             \
+    if (unlikely(size)) {                     \
+        if (unlikely(*size < sizeof(type))) { \
+            return UC_ERR_NOMEM;              \
+        }                                     \
+        *size = sizeof(type);                 \
+    }                                         \
+    ret = UC_ERR_OK;                          \
 } while(0)
 
 #endif