[MPEG2 Encode] Fix Coverity CID 4398024: INTEGER_OVERFLOW in VAAPIEncoder::SetFrames

In SetFrames, when m_RecFrameMemID is null, ind was set to 0xff (255).
This value is then assigned into CurrReconstructedPic.Index7Bits and
CurrOriginalPic.Index7Bits, both typed as UCHAR:7 (max 127), causing
Coverity to flag INTEGER_OVERFLOW (CWE-190).

Fix: use 0x7f as the invalid frame sentinel, which is the correct
maximum value for the 7-bit field and explicitly safe-by-construction.
No functional regression: hardware already truncated 0xff to 0x7f.

Author: stellawuintel

_studio/mfx_lib/ext/mpeg2/src/mfx_mpeg2_encode_vaapi.cpp

@@ -1406,7 +1406,7 @@ mfxStatus VAAPIEncoder::SetFrames (ExecuteBuffers* pExecuteBuffers)
     }
     else
     {
-        ind = 0xff;
+        ind = 0x7f; // max value for 7-bit Index7Bits field (invalid frame sentinel)
     }
 
     pExecuteBuffers->m_pps.CurrReconstructedPic.Index7Bits     =  mfxU8(ind < 0 ? 0 : ind);