internal: Introduce macro for stealing pointers

VIR_STEAL_PTR copies the pointer from the second argument into the
first argument and then sets the second to NULL.

src/internal.h

@@ -306,6 +306,18 @@
         (a) = (a) ^ (b);    \
     } while (0)
 
+/**
+ * VIR_STEAL_PTR:
+ *
+ * Steals pointer passed as second argument into the first argument. Second
+ * argument must not have side effects.
+ */
+# define VIR_STEAL_PTR(a, b)  \
+    do {                      \
+        (a) = (b);            \
+        (b) = NULL;           \
+    } while (0)
+
 /**
  * virCheckFlags:
  * @supported: an OR'ed set of supported flags