Optionally return bytes received from http_file_get()

`http_dl->bytes_received` was already free'd when it was accessed inside
`aesgcm_file_get()`.

Change `http_file_get()` to optionally return the number of bytes received
so we know how much data we have to decrypt.

This fixes #1994

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

src/tools/http_download.h

@@ -58,6 +58,7 @@ typedef struct http_download_t
     pthread_t worker;
     int cancel;
     gboolean silent;
+    gboolean return_bytes_received;
 } HTTPDownload;
 
 void* http_file_get(void* userdata);