pw_tokenizer: Fix decode string size type
pw_tokenizer: Fix decode string size type

Clang is unnecessarily strict with its narrowing rules, add a cast
to size_t to avoid using a '-Wno-c++11-narrowing', which could
potentially hide legitimate problems.

Change-Id: I87a9838816b68f69a1fe1d02b06a41d218d49fce
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98267
Pigweed-Auto-Submit: Karthik Bharadwaj <deef15c3af173cd1b11924c66bffc2b0847572cb@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed