fix: `round_up_to_page_size` overflow

Closes: #200 (closed)

The spec doesn't define which error code to set if len is too large. ENOMEM is the closest defined condition that fits:

[...] the range [addr,addr+len) exceeds that allowed for the address space of a process [...]

Logically, len would lead to addr exceeding the address space of a process if rounding it up to the next page size causes an overflow.