curl_sasl: Fixed potential null pointer utilisation

Although this should never happen due to the relationship between the
'mech' and 'resp' variables, and the way they are allocated together,
it does cause problems for code analysis tools:

V595 The 'mech' pointer was utilized before it was verified against
     nullptr. Check lines: 376, 381. curl_sasl.c 376

Bug: https://github.com/curl/curl/issues/745
Reported-by: Alexis La Goutte