BUILD: ist: prevent gcc11 maybe-uninitialized warning on istalloc

BUILD: ist: prevent gcc11 maybe-uninitialized warning on istalloc

A new warning is reported by gcc11 when using a pointer to uninitialized
memory block for a function with a const pointer argument. The warning
is triggered for istalloc, used by http_client.c / proxy.c / tcpcheck.c.

This warning is reported because the uninitialized memory block
allocated by malloc should not be passed to a const argument as in ist2.
See https://gcc.gnu.org/onlinedocs/gcc-11.1.0/gcc/Warning-Options.html#index-Wmaybe-uninitialized

This should be backported up to 2.2.