call progress callback at the beginning of pbkdf2 calculation

pbkdf2.c

@@ -42,6 +42,9 @@ void pbkdf2_hmac_sha256(const uint8_t *pass, int passlen, uint8_t *salt, int sal
 		salt[saltlen + 3] = i & 0xFF;
 		hmac_sha256(pass, passlen, salt, saltlen + 4, g);
 		memcpy(f, g, HMACLEN);
+		if (progress_callback) {
+			progress_callback(0, iterations);
+		}
 		for (j = 1; j < iterations; j++) {
 			hmac_sha256(pass, passlen, g, HMACLEN, g);
 			for (k = 0; k < HMACLEN; k++) {
@@ -77,6 +80,9 @@ void pbkdf2_hmac_sha512(const uint8_t *pass, int passlen, uint8_t *salt, int sal
 		salt[saltlen + 3] = i & 0xFF;
 		hmac_sha512(pass, passlen, salt, saltlen + 4, g);
 		memcpy(f, g, HMACLEN);
+		if (progress_callback) {
+			progress_callback(0, iterations);
+		}
 		for (j = 1; j < iterations; j++) {
 			hmac_sha512(pass, passlen, g, HMACLEN, g);
 			for (k = 0; k < HMACLEN; k++) {