Fixed <stdio.h> file and dereferencing bugs

[ZERICO2005]

Fixed the following issues:

[P2] clearerr indexes file streams off by one: src/libc/clearerr.c:13. fopen stores slot 1 at _file_streams[0], but clearerr uses _file_streams[stream->slot]; slot 5 writes past FOPEN_MAX.
[P2] fread(stdin, ...) reports full success after EOF/partial reads: src/libc/fread.c:20. The loop stops on EOF but returns count unconditionally instead of completed elements.
[P2] fwrite(stdout, size=0, ...) divides by zero: src/libc/fwrite.c:17. For zero-sized writes, the loop does nothing, then returns num / size.

Also fixed:

• Dereferencing FILE * stream when it is NULL, stdin, stdout, or stderr.
• return value for fseek and ftell