Fix errors pointed out by static analyzer

[Medvecrab]

A static analyzer found some errors -

1. Deref of Null - admin.c

Return value of a function 'pktbuf_dynamic' is dereferenced at admin.c:245 without checking for NULL, but it is usually checked for this function (25/26).

2. Memory Leak - proto.c

Dynamic memory, referenced by 'ibuf', is allocated at proto.c:474 by calling function 'malloc' and lost at proto.c:488.

3. Memory Leak - scram.c

Dynamic memory, referenced by 'server_signature_base64', is allocated at scram.c:932 by calling function 'malloc' and lost at scram.c:938.

4. No VA End - proto.c : 686;

Function 'va_end' was not called before proto.c:686 inside function 'scan_text_result'. (Function 'va_start' was called at proto.c:624.)

5. No VA End - proto.c : 693

Function 'va_end' was not called before proto.c:693 inside function 'scan_text_result'. (Function 'va_start' was called at proto.c:624.)

So here's proposed fixes

[JelteF]

To avoid mistakes like this in the future, let's use the goto failed pattern for this function.

[JelteF]

Thank you, looks good overall. But left one small suggestion for an improvement.

[Medvecrab]

Added suggested change, thanks for review!

[JelteF]

FYI the formatting check is failing, make format-c should fix that.

[Medvecrab]

Force-pushed a formatting fix