usb-mtp: fix utf16_to_str

Make utf16_to_str return an allocated string.  Remove the assumtion that
the number of string bytes equals the number of utf16 chars (which is
only true for ascii chars).  Instead call wcstombs twice, once to figure
the storage size and once for the actual conversion (as suggested by the
wcstombs manpage).

FIXME: surrogate pairs are not working correctly.  Pre-existing bug,
fixing that is left for another day.
Reported-by: NMichael Hanselmann <public@hansmi.ch>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-id: 20181203101045.27976-2-kraxel@redhat.com

hw/usb/dev-mtp.c

@@ -1593,17 +1593,23 @@ static void usb_mtp_cancel_packet(USBDevice *dev, USBPacket *p)
     fprintf(stderr, "%s\n", __func__);
 }
 
-static void utf16_to_str(uint8_t len, uint16_t *arr, char *name)
+static char *utf16_to_str(uint8_t len, uint16_t *arr)
 {
-    int count;
-    wchar_t *wstr = g_new0(wchar_t, len);
+    wchar_t *wstr = g_new0(wchar_t, len + 1);
+    int count, dlen;
+    char *dest;
 
     for (count = 0; count < len; count++) {
+        /* FIXME: not working for surrogate pairs */
         wstr[count] = (wchar_t)arr[count];
     }
+    wstr[count] = 0;
 
-    wcstombs(name, wstr, len);
+    dlen = wcstombs(NULL, wstr, 0) + 1;
+    dest = g_malloc(dlen);
+    wcstombs(dest, wstr, dlen);
     g_free(wstr);
+    return dest;
 }
 
 /* Wrapper around write, returns 0 on failure */
@@ -1703,7 +1709,7 @@ static void usb_mtp_write_metadata(MTPState *s)
 {
     MTPData *d = s->data_out;
     ObjectInfo *dataset = (ObjectInfo *)d->data;
-    char *filename = g_new0(char, dataset->length);
+    char *filename;
     MTPObject *o;
     MTPObject *p = usb_mtp_object_lookup(s, s->dataset.parent_handle);
     uint32_t next_handle = s->next_handle;
@@ -1711,7 +1717,7 @@ static void usb_mtp_write_metadata(MTPState *s)
     assert(!s->write_pending);
     assert(p != NULL);
 
-    utf16_to_str(dataset->length, dataset->filename, filename);
+    filename = utf16_to_str(dataset->length, dataset->filename);
 
     o = usb_mtp_object_lookup_name(p, filename, dataset->length);
     if (o != NULL) {