pw_string: Fix dead initialization in tests
pw_string: Fix dead initialization in tests

Change-Id: Ie206705a78db14eb246ce80f1aff350942656aa5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/35561
Pigweed-Auto-Submit: Prashanth Swaminathan <7074f112da8ec89069105f3b18a4ce46a4722488@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

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