Left out valid end-of-string comparison in r30073.

Refs trac:4031

This commit was SVN r30074.

The following SVN revision numbers were found above:
  r30073 --> open-mpi/ompi@6003702a51

The following Trac tickets were found above:
  Ticket 4031 --> https://svn.open-mpi.org/trac/ompi/ticket/4031

ompi/mca/btl/usnic/btl_usnic_component.c

@@ -318,7 +318,7 @@ static int parse_vendor_part_ids(void)
     for (i = 0; !isspace(str[i]) && '\0' != str[i]; ++i) {
         continue;
     }
-    if (!isdigit(str[i])) {
+    if ('\0' != str[i] && !isdigit(str[i])) {
         ret = OMPI_ERR_BAD_PARAM;
         goto out;
     }