segfault when using libmagic and an empty buffer in file-4.16

[Mike Frysinger]

thought i already sent this but i must of forgotten

if you call magic_buffer(m, NULL, 0) it'll segfault on you because 
file_ascmagic() assumes that the count is always at least 2 bytes.  once this 
has been fixed, magic_buffer(m, NULL, 0) will return NULL instead of 
"empty" ...

so, the first fix is:
--- src/ascmagic.c
+++ src/ascmagic.c
@@ -179,6 +179,9 @@ file_ascmagic(struct magic_set *ms, cons
                }
        }
 
+       if (nbytes <= 1)
+               goto done;
+
        if ((*buf == 'c' || *buf == 'C') && ISSPC(buf[1])) {
                subtype_mime = "text/fortran";
                subtype = "fortran program";

while the second fix should be something like this i think:
file_buffer(struct magic_set *ms, int fd, const void *buf, size_t nb)
...
            /* abandon hope, all ye who remain here */
            if (file_printf(ms, ms->flags & MAGIC_MIME ?
                (nb ? "application/octet-stream" : "application/empty") :
                (nb ? "data" : "empty")) == -1)
                return -1;
...
-mike