Avoid left-shifting negative values to fix UBSAN warning

Instead, shift as an unsigned value, then convert back to signed:

```
    ../3rdParty/jxrlib/image/decode/segdec.c:1081:36: runtime error: left shift of negative value -1
    #0 0x7f0cc5c997c8 in DecodeMacroblockLowpass ../3rdParty/jxrlib/image/decode/segdec.c:1081
    #1 0x7f0cc5c2f4f4 in processMacroblockDec ../3rdParty/jxrlib/image/decode/strdec.c:417
    #2 0x7f0cc5c881f8 in ImageStrDecDecode ../3rdParty/jxrlib/image/decode/strdec.c:4010
    #3 0x7f0cc5b82102 in PKImageDecode_Copy_WMP ../3rdParty/jxrlib/jxrgluelib/JXRGlueJxr.c:1874
```

Co-authored-by: Milian Wolff <[email protected]>

image/decode/segdec.c

@@ -961,7 +961,7 @@ Int DecodeMacroblockLowpass (CWMImageStrCodec * pSC, CCodingContext *pContext,
                         pCoeffs[k] += getBits (pIO, iModelBits);
                     }
                     else if (pCoeffs[k] < 0) {
-                        pCoeffs[k] <<= iModelBits;
+                        pCoeffs[k] = (int) (((unsigned) pCoeffs[k]) << iModelBits);
                         pCoeffs[k] -= getBits (pIO, iModelBits);
                     }
 #endif // WIN32