test-fileio: do not use variable before checking return value

Coverity is unhappy because we use "line" in the assert that checks
the return value. It doesn't matter much, but let's clean this up.
Also, let's not assume that /proc/cmdline contains anything.

CID #1400219.

src/test/test-fileio.c

@@ -759,7 +759,11 @@ static void test_read_line3(void) {
         assert_se(f);
 
         r = read_line(f, LINE_MAX, &line);
-        assert_se((size_t) r == strlen(line) + 1);
+        assert_se(r >= 0);
+        if (r == 0)
+                assert_se(line && isempty(line));
+        else
+                assert_se((size_t) r == strlen(line) + 1);
         assert_se(read_line(f, LINE_MAX, NULL) == 0);
 }