convert: fix declarations after case label

This fixes a build failure with GCC/Clang where declarations directly after a case label are not valid in C.

Repro (Debian 11, GCC): make convert fails with: error: a label can only be part of a statement and a declaration is not a statement

Also reproducible with clang on macOS.

Fix: wrap the 'p' case body in braces so the declarations are within a block scope. Tested: make convert.