avifil32: Sign-compare warnings fix

[Andrew Talbot]

Changelog:
    avifil32: Sign-compare warnings fix.

diff --git a/dlls/avifil32/api.c b/dlls/avifil32/api.c
index 12eb1b7..5eccbdb 100644
--- a/dlls/avifil32/api.c
+++ b/dlls/avifil32/api.c
@@ -937,9 +937,9 @@ LONG WINAPI AVIStreamSampleToTime(PAVISTREAM pstream, LONG lSample)
     return -1;
 
   /* limit to stream bounds */
-  if (lSample < asiw.dwStart)
+  if ((DWORD)lSample < asiw.dwStart)
     lSample = asiw.dwStart;
-  if (lSample > asiw.dwStart + asiw.dwLength)
+  if ((DWORD)lSample > asiw.dwStart + asiw.dwLength)
     lSample = asiw.dwStart + asiw.dwLength;
 
   if (asiw.dwRate / asiw.dwScale < 1000)