Add assert()s to the implementation of xRead() in the built-in VFSes to

verify that the offset parameter is always non-negative.

FossilOrigin-Name: cf5c3642247fdd34d87f0368594cd7b8f081636a

src/os_unix.c

@@ -3152,6 +3152,8 @@ static int unixRead(
   unixFile *pFile = (unixFile *)id;
   int got;
   assert( id );
+  assert( offset>=0 );
+  assert( amt>0 );
 
   /* If this is a database file (not a journal, master-journal or temp
   ** file), the bytes in the locking range should never be read or written. */