sampling : fix use-after-scope in grammar trigger_words path (#2221)

llama_sampler_init_grammar_impl built the trigger_words pattern in a block-scoped std::string, stored a pointer into it (trigger_pattern_c), pointed trigger_patterns at that pointer, then let the block drop both locals before llama_grammar_init_impl dereferenced trigger_patterns -- a read of a dangling pointer into freed std::string storage (UB). Hoist trigger_pattern and trigger_pattern_c to function scope so both outlive the call.

src/llama-sampling.cpp

@@ -1367,11 +1367,13 @@ struct llama_grammar* llama_sampler_init_grammar_impl(
     size_t num_trigger_patterns) {
     // Huh? this is not used and leaks. auto* ctx = new llama_sampler_grammar;
     struct llama_grammar* grammar;
+    std::string trigger_pattern;
+    const char * trigger_pattern_c = nullptr;
     if (grammar_str != nullptr && grammar_str[0] != '\0') {
         // TODO: remove trigger_words support.
         if (trigger_words != nullptr && num_trigger_words > 0) {
             GGML_ASSERT(trigger_patterns == nullptr && num_trigger_patterns == 0);
-            std::string trigger_pattern("[\\s\\S]*?(");
+            trigger_pattern = "[\\s\\S]*?(";
             for (size_t i = 0; i < num_trigger_words; ++i) {
                 static const std::regex special_chars("[.^$|()*+?\\[\\]{}\\\\]");
                 if (i > 0) {
@@ -1380,7 +1382,7 @@ struct llama_grammar* llama_sampler_init_grammar_impl(
                 trigger_pattern += std::regex_replace(trigger_words[i], special_chars, "\\$0");
             }
             trigger_pattern += ")[\\s\\S]*";
-            auto trigger_pattern_c = trigger_pattern.c_str();
+            trigger_pattern_c = trigger_pattern.c_str();
             trigger_patterns = &trigger_pattern_c;
             num_trigger_patterns = 1;
         }