fix(rust): Check dtvcc_rust instead of dtvcc in ccxr_process_cc_data

When Rust CEA-708 decoder is enabled, dec_ctx.dtvcc is set to NULL
and dec_ctx.dtvcc_rust holds the actual DtvccRust context. The null
check was incorrectly checking dtvcc, causing the function to return
early and skip all CEA-708 data processing.

This fixes tests 21, 31, 32, 105, 137, 141-149 which were failing
with exit code 10 (EXIT_NO_CAPTIONS) because no captions were being
extracted from CEA-708 streams.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cfsmp3

src/rust/src/lib.rs

@@ -392,8 +392,9 @@ extern "C" fn ccxr_process_cc_data(
 
     let dec_ctx = unsafe { &mut *dec_ctx };
 
-    // Check dtvcc pointer before dereferencing
-    if dec_ctx.dtvcc.is_null() {
+    // Check dtvcc_rust pointer before dereferencing (not dtvcc!)
+    // When Rust is enabled, dtvcc is NULL and dtvcc_rust holds the actual context
+    if dec_ctx.dtvcc_rust.is_null() {
         return -1;
     }