msi: Use mi->source if the source is not a full path.

dlls/msi/files.c

@@ -483,6 +483,9 @@ static UINT download_remote_cabinet(MSIPACKAGE *package, struct media_info *mi)
 
     *(ptr + 1) = '\0';
     ptr = strrchrW(mi->source, '\\');
+    if (!ptr)
+        ptr = mi->source;
+
     src = msi_realloc(src, (lstrlenW(src) + lstrlenW(ptr)) * sizeof(WCHAR));
     if (!src)
         return ERROR_OUTOFMEMORY;