Fixed subtle barrier_fpa bug in bli_thrcomm.c. (#690)

Details:
- In bli_thrcommo.c, correctly initialize the BLIS_OPENMP element of the
  barrier function pointer array (barrier_fpa) to NULL when
  BLIS_ENABLE_OPENMP is *not* defined. Similarly, initialize the
  BLIS_POSIX element of barrier_fpa to NULL when BLIS_ENABLE_PTHREADS is
  not enabled. This bug was introduced in a1a5a9b and was likely the
  result of an incomplete edit. The effects of the bug would have
  likely manifested when querying a thrcomm_t that was initialized with
  a timpl_t value corresponding to a threading implementation that was
  omitted from the -t option at configure-time.

Author: fgvanzee

frame/thread/bli_thrcomm.c

@@ -115,17 +115,17 @@ static thrcomm_barrier_ft barrier_fpa[ BLIS_NUM_THREAD_IMPLS ] =
 #if   defined(BLIS_ENABLE_OPENMP)
 	                bli_thrcomm_barrier_openmp,
 #elif defined(BLIS_ENABLE_PTHREADS)
-	                bli_thrcomm_barrier_pthreads,
+	                NULL,
 #else
-	                bli_thrcomm_barrier_single,
+	                NULL,
 #endif
 	[BLIS_POSIX]  =
 #if   defined(BLIS_ENABLE_PTHREADS)
 	                bli_thrcomm_barrier_pthreads,
 #elif defined(BLIS_ENABLE_OPENMP)
-	                bli_thrcomm_barrier_openmp,
+	                NULL,
 #else
-	                bli_thrcomm_barrier_single,
+	                NULL,
 #endif
 };