shell: don't ignore IO errors

sys/shell/shell.c

@@ -217,6 +217,9 @@ static int readline(shell_t *shell, char *buf, size_t size)
         }
 
         c = shell->readchar();
+        if (c < 0) {
+            return 1;
+        }
         shell->put_char(c);
 
         /* We allow Unix linebreaks (\n), DOS linebreaks (\r\n), and Mac linebreaks (\r). */

tests/test_shell/main.c

@@ -53,7 +53,10 @@ static void print_echo(int argc, char **argv)
 static int shell_readc(void)
 {
     char c;
-    posix_read(uart0_handler_pid, &c, 1);
+    int result = posix_read(uart0_handler_pid, &c, 1);
+    if (result != 1) {
+        return -1;
+    }
     return (unsigned char) c;
 }