cpio: Fix zero-length files

If c_filesize is zero, bail out immediately from parse_one(). Otherwise
the rdbuf()->pubsetbuf() call will wreak havoc on the stream position
and cause the next header to have corrupt CPIO magic.

Found by streaming openjdk rpm2cpio output to OSv.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Cc: Benoît Canet <benoit@irqsave.net>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

tools/cpiod/cpio.cc

@@ -81,6 +81,9 @@ bool cpio_in::parse_one(istream& is, cpio_in& out)
         return false;
     }
     auto filesize = convert(h.c_filesize);
+    if (!filesize) {
+        return true;
+    }
     auto aligned_filesize = align_up(filesize, 4u);
 
     auto file_slice = io::restrict(is, 0, filesize);