fix(process): out-of-bounds write when reading child process output

[jamofer]

read() can return up to buffer.size() (4096) bytes, so buffer[read_bytes + 1] = '\0' wrote past the end of the 4096-byte stack array — a stack buffer overflow when a child filled the buffer. Under ASan this surfaces as stack-buffer-overflow; without it, undefined behavior.

Fix: append exactly read_bytes bytes to the output string and drop the manual null terminator. Also stop the read loop on error (read() returning -1) instead of looping forever.

[cegonse]

Thanks a lot @jamofer ! Merged!