Fix wc -c optimization.

Check the fstat(2) return value rather than read uninitialized memory if
it failed, and add a special case for files that claim to be zero-length
but aren't (as is common in /proc on Linux).

toys/posix/wc.c

@@ -53,8 +53,8 @@ static void do_wc(int fd, char *name)
  if (toys.optflags == FLAG_c) {
  struct stat st;
 
- fstat(fd, &st);
- if (S_ISREG(st.st_mode)) {
+ // On Linux, files in /proc often report their size as 0.
+ if (!fstat(fd, &st) && S_ISREG(st.st_mode) && st.st_size > 0) {
  lengths[2] = st.st_size;
  goto show;
  }