mmap invalid file contents

[trufae]

we may be switching to stdio by default because mmap is known to be flacky way to access file contents, and also adding guardrails just make things slower without really fixing the toctou problem, but anyway, here's one testcase that shows how stdio works well but mmap misbehaves and both plugins must behave the same way:

$ echo hello world | tee a
hello world
$ r2 -qwc'wtf a $$@ 5;px 32' a
INFO: Dumped 5 bytes from 0x00000005 into a
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00000000  2077 6f72 6c00 0000 0000 0000 ffff ffff   worl...........
0x00000010  ffff ffff ffff ffff ffff ffff ffff ffff  ................

$ echo hello world | tee a
hello world
$ r2 -qwc'wtf a $$@ 5;px 32' stdio://a
INFO: Dumped 5 bytes from 0x00000005 into a
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00000000  2077 6f72 6cff ffff ffff ffff ffff ffff   worl...........
0x00000010  ffff ffff ffff ffff ffff ffff ffff ffff  ................
$