httpc: replace ibuf_alloc with xibuf_alloc

There is no check for NULL for a value returned by `ibuf_alloc`,
the NULL will be passed to `memcpy()` if the aforementioned
function will return a NULL. The patch fixes that by replacing
`ibuf_alloc` with macros `xibuf_alloc` that never return NULL.

Found by Svace.

NO_CHANGELOG=codehealth
NO_DOC=codehealth
NO_TEST=codehealth

(cherry picked from commit b4ee146fde6e418aed590ac6054cff75c2a59626)