Handle cases where a model does not repeat the function signature

[XZ-X]

This PR attempts to handle cases where a model does not repeat the function signature.

Note that the variable start denotes the position of the queried function signature, and it will be set to 0 the function signature is not found in a model's response.

Suppose that the query prompt is

// query prefix here
void strlen(string s){

If a model returns

    return s.length();
}

The original processing code yields

// query prefix here
    return s.length();

which fails to compile.

A possible fix (as proposed by this PR) is not to skip the function signature when it's not found in a model's response.
That is, instead of returning func_prefix + '\n' + body + '\n', the function returns question + '\n' + body when the signature is not found.