fix UB and possible exception trigger

[scaprile]

ASAN was reporting undefined behaviour (UB).

src/net_builtin.c:655:37: runtime error: applying zero offset to null pointer

Weird as it seems, &io->buf[io->len] is UB in C and in C++ < C++17 for a NULL pointer, even if the offset is 0. This condition happens at TLS receive start time, when TLS has not yet decrypted any record and hence no iobuf has been created for decrypted data.

Once this is fixed by eliminating the offset when the pointer is NULL, then the first time a record gets decrypted there is no buffer to hold data, it will be created at the next loop. when mg_tls_avail() will be called. We then pass a NULL destination pointer to memmove(), which, even though len is 0, will violate the specs and trigger ASAN again

src/tls_builtin.c:1385:11: runtime error: null pointer passed as argument 1, which is declared to never be null
/usr/include/string.h:48:14: note: nonnull attribute specified here

So, now we just return 0 when either the buffer pointer is NULL or buffer length is 0