fpurge: Make it work with non fd based FILEs

Streams opened with for example open_memstream(3) will have no associated
file descriptor, and fileno(3) will fail.

Signed-off-by: Guillem Jover <guillem@hadrons.org>

src/fpurge.c

@@ -32,7 +32,7 @@
 int
 fpurge(FILE *fp)
 {
-	if (fp == NULL || fileno(fp) < 0) {
+	if (fp == NULL) {
 		errno = EBADF;
 		return EOF;
 	}