(smile) Handle sequence of Smile header markers without recursion

[cowtowncoder]

(found by ossfuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32665)

A sequence of 4-byte Smile header markers is technically legal (even if generally useless), and the current handling that uses recursion can be problematic if caller feeds parser a very long sequence (in thousands of markers).
Code should be changed to avoid recursive calls.