lib/string_helpers: rework overflow-dependent code

When @size is 0, the desired behavior is to allow unlimited bytes to be
parsed. Currently, this relies on some intentional arithmetic overflow
where --size gives us SIZE_MAX when size is 0.

Explicitly spell out the desired behavior without relying on intentional
overflow/underflow.

Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20240808-b4-string_helpers_caa133-v1-1-686a455167c4@google.com


Signed-off-by: Kees Cook <kees@kernel.org>