Fix -Wunreachable-code warning in malloc_vsnprintf().

Variables s and slen are declared inside a switch statement, but outside
a case scope. clang reports these variable definitions as "unreachable",
though this is not really meaningful in this case. This is the only
-Wunreachable-code warning in jemalloc.

src/util.c:501:5 [-Wunreachable-code] code will never be executed

This resolves #364.