terminal-util: correct fd validity check

Follow-up for 14f594b995

src/basic/terminal-util.c

@@ -146,7 +146,7 @@ int read_one_char(FILE *f, char *ret, usec_t t, bool *need_nl) {
                 }
         }
 
-        if (t != USEC_INFINITY && fd > 0) {
+        if (t != USEC_INFINITY && fd >= 0) {
                 /* Let's wait the specified amount of time for input. When we have no fd we skip this, under
                  * the assumption that this is an fmemopen() stream or so where waiting doesn't make sense
                  * anyway, as the data is either already in the stream or cannot possible be placed there