allow fmemopen with zero size

previously, POSIX erroneously required this to fail with EINVAL
despite the traditional glibc implementation, on which the POSIX
interface was based, allowing it. the resolution of Austin Group issue
818 removes the requirement to fail.

src/stdio/fmemopen.c

@@ -83,7 +83,7 @@ FILE *fmemopen(void *restrict buf, size_t size, const char *restrict mode)
 	struct mem_FILE *f;
 	int plus = !!strchr(mode, '+');
 	
-	if (!size || !strchr("rwa", *mode)) {
+	if (!strchr("rwa", *mode)) {
 		errno = EINVAL;
 		return 0;
 	}