refactor(math): use explicit select case for diff shape allocation

Improved the boundary condition handling in diff_2 functions by
using an explicit select case for dimensions. This ensures that:
1. Shape preservation logic is clear.
2. Invalid dimension values (dim_ != 1 or 2) trigger a descriptive error stop.

Author: JAi-SATHVIK

src/math/stdlib_math_diff.fypp

@@ -91,11 +91,14 @@ contains
         size_work = size_x + size_prepend + size_append
 
         if (size_work <= n_) then
-            if (dim_ == 1) then
+            select case (dim_)
+            case (1)
                 allocate(y(0, size(x, 2)))
-            else
+            case (2)
                 allocate(y(size(x, 1), 0))
-            end if
+            case default
+                error stop "diff_2: invalid dimension (dim_ must be 1 or 2)"
+            end select
             return
         end if