pw_string: Initialize values in to_string_test

CustomType was used uninitialized in tests. Its uninitialized members
were never accessed, but this triggered -Wmaybe-uninitialized errors in
GCC 11.

Change-Id: I7dec5bddf20895adfef1ed80e1cd818b3f2a3f1f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/111072
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
diff --git a/pw_string/to_string_test.cc b/pw_string/to_string_test.cc
index 289b068..39a09fb 100644
--- a/pw_string/to_string_test.cc
+++ b/pw_string/to_string_test.cc
@@ -33,7 +33,7 @@
 
   static constexpr const char* kToString = "This is a CustomType";
 
-  CustomType() = default;
+  CustomType() : a(0), b(0) {}
 
   // Non-copyable to verify that ToString doesn't copy it.
   CustomType(const CustomType&) = delete;