Fix compilation warning in bcast.c

Fixes the following warning when --enable-fast is enabled:

src/mpi/coll/bcast.c:1012:23: warning: suggest braces around empty body
in an 'if' statement [-Wempty-body]

See #1966.
Signed-off-by: Pavan Balaji <balaji@mcs.anl.gov>

src/mpi/coll/bcast.c

@@ -1008,8 +1008,9 @@ static int MPIR_SMP_Bcast(
     MPI_Status status;
     int recvd_size;
 
-    if (!MPIR_CVAR_ENABLE_SMP_COLLECTIVES || !MPIR_CVAR_ENABLE_SMP_BCAST)
+    if (!MPIR_CVAR_ENABLE_SMP_COLLECTIVES || !MPIR_CVAR_ENABLE_SMP_BCAST) {
         MPIU_Assert(0);
+    }
     MPIU_Assert(MPIR_Comm_is_node_aware(comm_ptr));
 
     is_homogeneous = 1;