Fix seg fault when fd_order is on (MFlowCode#704)

Author: haochey

src/simulation/m_global_parameters.fpp

@@ -1015,6 +1015,10 @@ contains
             buff_size = weno_polyn + 2
         end if
 
+        if (probe_wrt) then
+            fd_number = max(1, fd_order/2)
+        end if
+
         ! Configuring Coordinate Direction Indexes =========================
         idwint(1)%beg = 0; idwint(2)%beg = 0; idwint(3)%beg = 0
         idwint(1)%end = m; idwint(2)%end = n; idwint(3)%end = p
@@ -1037,11 +1041,6 @@ contains
                 & idwbuff(3)%beg:idwbuff(3)%end))
         end if
 
-        if (probe_wrt) then
-            fd_number = max(1, fd_order/2)
-            buff_size = buff_size + fd_number
-        end if
-
         startx = -buff_size
         starty = 0
         startz = 0