Fix macOS segfault when a page does not exist

[NNoradrenaline]

What does it do?

Fixes an out-of-bounds write in curl_string_init() that can crash the client when a requested page does not exist and the code falls back to the network path.

The response buffer is now initialized with str->str[0] = '\0'; instead of writing one byte past the 1024-byte allocation.

This also adds regression coverage for missing pages and a macOS AddressSanitizer CI job.

Why the change?

Issues #124 and #123 report crashes on macOS when requesting unknown or unavailable pages.

curl_string_init() allocates 1024 bytes, but previously wrote str->str[1024] = '\0';.

Valid indexes are 0 through 1023, so this write is out of bounds and can cause the reported crash.

How can this be tested?

Build the client and run:

./tldr this-page-definitely-does-not-exist-124-regression

Expected behavior:

• The client prints the normal missing-page message.
• It exits with status 1.
• It does not crash.

The added CI coverage also exercises this path under AddressSanitizer on macOS.

Where to start code review?

Start in src/net.c at curl_string_init().

The regression test and sanitizer configuration are in .github/workflows/tests.yml.

Relevant tickets?

Fixes #124

Related: #123

Questions?

None.