fix warning "ignoring return value" of fwrite.

Merge pull request #343

contributed by Eric Herman.

sql/wsrep_binlog.cc

@@ -309,9 +309,13 @@ void wsrep_dump_rbr_buf(THD *thd, const void* rbr_buf, size_t buf_len)
   }
 
   FILE *of= fopen(filename, "wb");
+
   if (of)
   {
-    fwrite (rbr_buf, buf_len, 1, of);
+    if (fwrite(rbr_buf, buf_len, 1, of) == 0)
+       WSREP_ERROR("Failed to write buffer of length %llu to '%s'",
+                   (unsigned long long)buf_len, filename);
+
     fclose(of);
   }
   else