avoid overflow in computing mid index

Author: gareth-nx

src/stdlib_selection.fypp

@@ -91,7 +91,7 @@ contains
           end if
 
           do while(.true.)
-              mid = (l+r)/2_ip ! Deliberate integer division
+              mid = l + (r-l)/2_ip ! Deliberate integer division
 
               call medianOf3(a, l, mid, r)
               call swap(a(l), a(mid))
@@ -207,7 +207,7 @@ contains
           end if
 
           do while(.true.)
-              mid = (l+r)/2_ip ! Deliberate integer division
+              mid = l + (r-l)/2_ip ! Deliberate integer division
 
               call arg_medianOf3(a, indx, l, mid, r)
               call swap(indx(l), indx(mid))