build: Pacify GCC 5 on 32-bit hosts

This pacifies GCC 5 in a better way, without disabling diagnostics.
* src/df.c (main): Tell compiler that optind is positive.
* src/shred.c (known): New function.
(dopass): Go back to off_t for file sizes.
Avoid integer overflow if we run off the end of the file.
Tell compiler that a write cannot write more bytes than requested.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/df.c b/src/df.c
index 17a2de437..507a0f734 100644
--- a/src/df.c
+++ b/src/df.c
@@ -1650,6 +1650,8 @@ main (int argc, char **argv)
       return EXIT_FAILURE;
   }
 
+  assume (0 < optind);
+
   if (optind < argc)
     {
       int i;