fileio: fail early if we can't return the number of bytes we read anymore in an int

This is mostly paranoia, but let's better be safer than sorry. This of
course means there's always an implicit limit to how much we can read at
a time of 2G. But that should be ample.

src/basic/fileio.c

@@ -762,6 +762,9 @@ int read_line(FILE *f, size_t limit, char **ret) {
                         if (n >= limit)
                                 return -ENOBUFS;
 
+                        if (count >= INT_MAX) /* We couldn't return the counter anymore as "int", hence refuse this */
+                                return -ENOBUFS;
+
                         errno = 0;
                         c = fgetc_unlocked(f);
                         if (c == EOF) {