Corrected a max to a min in i2s

  A recent update intended to add a min of n and 3 to avoid segmentation faults,
but a max was added instead, triggering segmentation faults.  All answers are
still bitwise identical in the test cases, but this now passes a more sensitive
compiler's checks.

src/framework/MOM_diag_mediator.F90

@@ -2562,7 +2562,7 @@ function i2s(a,n_in)
   if (present(n_in)) n = n_in
 
   i2s = ''
-  do i=1,max(n,3)
+  do i=1,min(n,3)
     write (i2s_temp, '(I4.4)') a(i)
     i2s = trim(i2s) //'_'// trim(i2s_temp)
   enddo