reset n before stderr path too

core/os/os2/process.odin

@@ -426,6 +426,7 @@ process_exec :: proc(
 			}
 
 			if err == nil && !stderr_done {
+				n = 0
 				has_data, err = pipe_has_data(stderr_r)
 				if has_data {
 					n, err = read(stderr_r, buf[:])

tests/core/os/os2/process.odin

@@ -21,4 +21,5 @@ test_process_exec :: proc(t: ^testing.T) {
 	testing.expect_value(t, state.success, true)
 	testing.expect_value(t, err, nil)
 	testing.expect_value(t, string(stdout), "hellope\n")
+	testing.expect_value(t, string(stderr), "")
 }