Error check qoi_write() more strictly

simply checking the return value of fwrite() wouldn't be enough since
stdio is typically buffered. and so force a flush and check for errors
via ferror().

qoi.h

@@ -594,7 +594,7 @@ void *qoi_decode(const void *data, int size, qoi_desc *desc, int channels) {
 
 int qoi_write(const char *filename, const void *data, const qoi_desc *desc) {
 	FILE *f = fopen(filename, "wb");
-	int size;
+	int size, err;
 	void *encoded;
 
 	if (!f) {
@@ -608,10 +608,12 @@ int qoi_write(const char *filename, const void *data, const qoi_desc *desc) {
 	}
 
 	fwrite(encoded, 1, size, f);
+	fflush(f);
+	err = ferror(f);
 	fclose(f);
 
 	QOI_FREE(encoded);
-	return size;
+	return err ? 0 : size;
 }
 
 void *qoi_read(const char *filename, qoi_desc *desc, int channels) {