Add work-around for MPICH 3.3 defect.

src/pio_posixfpguardsendrecv.c

@@ -338,12 +338,14 @@ initSharedFP(struct sharedFP *bfd, int nProcsColl)
 
 /* MPICH 3.3 has a problem in its implementation of MPI_Testany and
  * MPI_Waitany, see commit 0f7be7196cc05bf0c908761e148628e88d635190
- * at https://github.com/pmodels/mpich */
+ * at https://github.com/pmodels/mpich
+ * effectively, the work-around prepends MPI_REQUEST_NULL to the
+ * request array */
 #ifdef MPICH_CALC_VERSION
 #if MPICH_NUMVERSION >= MPICH_CALC_VERSION(3, 3, 0, 0, 0) && MPI_NUMVERSION < MPICH_CALC_VERSION(3, 3, 1, 0, 0)
 #define CDIPIO_MPICH33_WORKAROUND(code) code
 #else
-#define CDIPIO_MPICH33_WORKAROUND(code) code
+#define CDIPIO_MPICH33_WORKAROUND(code)
 #endif
 #else
 #define CDIPIO_MPICH33_WORKAROUND(code)