mca_base_var.c: add debug check for another programming error

Coverity alerted us to the fact that there are places where
the synonym_for param is hard-coded to -1 when calling
register_variable().  It would be a coding error if synonym_for==-1
and (flags & MCA_BASE_VAR_FLAG_SYNONYM)>0, so let's add that to the
debug-only check at the top of the function.

This was CID 993717.

opal/mca/base/mca_base_var.c

@@ -10,7 +10,7 @@
  *                         University of Stuttgart.  All rights reserved.
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
- * Copyright (c) 2008-2014 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2008-2015 Cisco Systems, Inc.  All rights reserved.
  * Copyright (c) 2012-2014 Los Alamos National Security, LLC. All rights
  *                         reserved.
  * Copyright (c) 2014      Intel, Inc. All rights reserved.
@@ -1244,6 +1244,12 @@ static int register_variable (const char *project_name, const char *framework_na
     if (0 != align) {
         assert(((uintptr_t) storage) % align == 0);
     }
+
+    /* Also check to ensure that synonym_for>=0 when
+       MCA_BCASE_VAR_FLAG_SYNONYM is specified */
+    if (flags & MCA_BASE_VAR_FLAG_SYNONYM && synonym_for < 0) {
+        assert((flags & MCA_BASE_VAR_FLAG_SYNONYM) && synonym_for >= 0);
+    }
 #endif
 
     /* There are data holes in the var struct */