Initialise buffer locals at declaration

[chrisbazley]

Summary

• initialise out_buffer and out_size when they are declared
• initialise in_buffer and in_size when they are declared
• preserve their scope through the remainder of their respective functions

The existing 21-Sep-26 history entries already describe these follow-up corrections.

Validation

• CMake build succeeded
• GKeyTests passed
• git diff --check passed