Fix an issue where _IONBF would cause an overflow error

src/stdio/src/lib.rs

@@ -99,7 +99,11 @@ impl FILE {
             return true;
         }
         self.read = None;
-        self.write = Some((self.unget, self.unget, self.buf.len() - 1));
+        self.write = if self.buf.len() == 0 {
+            Some((0, 0, 0))
+        } else {
+            Some((self.unget, self.unget, self.buf.len() - 1))
+        };
         return true;
     }
     pub fn write(&mut self, to_write: &[u8]) -> usize {

tests/stdio/all.c

@@ -7,5 +7,7 @@ int main(int argc, char ** argv) {
 	ungetc('H', f);
 	char *in = malloc(30);
 	printf("%s\n", fgets(in, 30, f));
+	setvbuf(stdout, 0, _IONBF, 0);
+	printf("Hello\n");
 	return 0;
 }