fix(math): add cross-dimension shape validation for diff prepend/append

Author: srinjoy933

src/math/stdlib_math_diff.fypp

@@ -85,8 +85,20 @@ contains
             dim_ = 1
         end if
 
-        if (present(prepend)) size_prepend = size(prepend, dim_)
-        if (present(append)) size_append = size(append, dim_)
+        if (present(prepend)) then
+            if (size(prepend, 3 - dim_) /= size(x, 3 - dim_)) then
+                error stop "stdlib_math_diff: non-differencing dimension of 'prepend' must match 'x'"
+            end if
+            size_prepend = size(prepend, dim_)
+        end if
+        
+        if (present(append)) then
+            if (size(append, 3 - dim_) /= size(x, 3 - dim_)) then
+                error stop "stdlib_math_diff: non-differencing dimension of 'append' must match 'x'"
+            end if
+            size_append = size(append, dim_)
+        end if
+        
         size_x = size(x, dim_)
         size_work = size_x + size_prepend + size_append