util: process: fix build on FreeBSD

Commit 825df8c3 refactored virProcess{Set,Get}Affinity routines,
however broke BSD implementation because of the incorrect variable
name. Fix build by using a proper variable name.

Pushing as trivial and build break fix.

src/util/virprocess.c

@@ -566,7 +566,7 @@ virProcessGetAffinity(pid_t pid)
         return NULL;
     }
 
-    if (!(*map = virBitmapNew(sizeof(mask) * 8)))
+    if (!(ret = virBitmapNew(sizeof(mask) * 8)))
         return NULL;
 
     for (i = 0; i < sizeof(mask) * 8; i++)