util: Umark virBufferCheckErrorInternal as ATTRIBUTE_RETURN_CHECK

The function is useful even without using the return value.  And if
needed, the return value can be obtained by other calls as well.  The
potential for clean-up can be seen in the following patch.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/util/virbuffer.h

@@ -58,7 +58,7 @@ int virBufferCheckErrorInternal(const virBuffer *buf,
                                 const char *filename,
                                 const char *funcname,
                                 size_t linenr)
-    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1);
+    ATTRIBUTE_NONNULL(1);
 /**
  * virBufferCheckError
  *