opal: minor update to guess_strlen

This is a minor update to
open-mpi/ompi@c52601f0c5.

If we have vsnprintf(), we might as well not have the rest of the
guess_strlen() routine.  Also document the nifty trick/behavior of
vsnprintf() that enables this shortcut (it was new to me!).

opal/util/printf.c

@@ -9,7 +9,7 @@
  *                         University of Stuttgart.  All rights reserved.
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
- * Copyright (c) 2007      Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2007-2014 Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  * 
  * Additional copyrights may follow
@@ -40,6 +40,13 @@
  */
 static int guess_strlen(const char *fmt, va_list ap)
 {
+#if HAVE_VSNPRINTF
+    char dummy[1];
+
+    /* vsnprintf() returns the number of bytes that would have been
+       copied if the provided buffer were infinite. */
+    return 1 + vsnprintf(dummy, sizeof(dummy), fmt, ap);
+#else
     char *sarg, carg;
     double darg;
     float farg;
@@ -48,13 +55,6 @@ static int guess_strlen(const char *fmt, va_list ap)
     int len;
     long larg;
 
-#if HAVE_VSNPRINTF
-    {
-        char dummy[1];
-        return 1 + vsnprintf(dummy, sizeof(dummy), fmt, ap);
-    }
-#endif
-    
     /* Start off with a fudge factor of 128 to handle the % escapes that
        we aren't calculating here */
 
@@ -190,6 +190,7 @@ static int guess_strlen(const char *fmt, va_list ap)
     }
 
     return len;
+#endif
 }