util: conf: Don't log when adding commented out lines

virConfAddEntry spams debug logs even for fully commented out lines.
Skip such messages to avoid:

2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
2017-05-12 12:35:38.867+0000: 10820: debug : virConfAddEntry:241 : Add entry (null) (nil)
...

This also fixes NULL passed to printf.

src/util/virconf.c

@@ -238,7 +238,10 @@ virConfAddEntry(virConfPtr conf, char *name, virConfValuePtr value, char *comm)
     if ((comm == NULL) && (name == NULL))
         return NULL;
 
+    /* don't log fully commented out lines */
+    if (name)
         VIR_DEBUG("Add entry %s %p", name, value);
+
     if (VIR_ALLOC(ret) < 0)
         return NULL;