common : fix SIGSEGV in _visit_pattern when quantifier follows empty seq

[pqhaz3925]

Problem

llama-server crashes with SIGSEGV when processing requests containing tool definitions with a JSON schema pattern like:

"pattern": "^$|^(?:-?\\d+[:-]-?\\d+)$"

This pattern is used in MCP tool servers (e.g. Figma MCP) for nodeId parameters. The crash happens on the first request that includes such tools.

Root Cause

In _visit_pattern(), the transform() lambda is called recursively when a ( is encountered. When the pattern contains (?:, the parser:

1. Sees (, increments i, sees ?, pushes a warning — but does not increment i past ?
2. Calls transform() recursively — seq is empty in the new frame
3. The recursive transform() immediately sees ? as the first character
4. Falls into the quantifier branch: seq.back() = ... — undefined behavior on empty vector → SIGSEGV

Fix

Add a guard before seq.back() in the quantifier case:

} else if (c == '*' || c == '+' || c == '?') {
    if (!seq.empty()) {
        seq.back() = std::make_pair(to_rule(seq.back()) + c, false);
    } else {
        _warnings.push_back("Quantifier without preceding element in pattern");
    }
    i++;

Reproduction

Any request to /v1/chat/completions with a tool schema containing "pattern": "^$|^(?:...)$".

GDB backtrace shows:

Thread received signal SIGSEGV
#0  common_schema_converter::_visit_pattern::{lambda#1}::operator()
#5  common_schema_converter::_visit_pattern
#6  common_schema_converter::visit
#7  common_schema_converter::_build_object_rule
#20 autoparser::peg_generator::generate_parser
#21 common_chat_templates_apply_jinja
#23 oaicompat_chat_params_parse

Tested and confirmed fixed on A100 SXM4-80GB (CUDA 12.6).