virBuffer: Try harder to free buffer

Currently, the way virBufferFreeAndReset() works is it relies on
virBufferContentAndReset() to fetch the buffer content which is
then freed. This works as long as there is no bug in virBuffer*
implementation (not true apparently). Explicitly call free() over
buffer content.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

src/util/virbuffer.c

@@ -281,9 +281,8 @@ virBufferContentAndReset(virBufferPtr buf)
  */
 void virBufferFreeAndReset(virBufferPtr buf)
 {
-    char *str = virBufferContentAndReset(buf);
-
-    VIR_FREE(str);
+    if (buf)
+        virBufferSetError(buf, 0);
 }
 
 /**