avdevice/dshow: Initialize 2 pointers

Coverity claims these are used uninitilaized in CID1598561 Uninitialized pointer write and CID1598565 Uninitialized pointer write

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavdevice/dshow.c

@@ -898,8 +898,8 @@ dshow_cycle_formats(AVFormatContext *avctx, enum dshowDeviceType devtype,
 
         if (devtype == VideoDevice) {
             VIDEO_STREAM_CONFIG_CAPS *vcaps = caps;
-            BITMAPINFOHEADER *bih;
-            int64_t *fr;
+            BITMAPINFOHEADER *bih = NULL;
+            int64_t *fr = NULL;
 #if DSHOWDEBUG
             ff_print_VIDEO_STREAM_CONFIG_CAPS(vcaps);
 #endif