Don't left-shift negative values to fix UBSAN warning

Instead, make the value positive, do the left-shift then, and make the
result negative again afterwards:

```
    ../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
    4creators#3 0x7f0cc5b82102 in PKImageDecode_Copy_WMP ../3rdParty/jxrlib/jxrgluelib/JXRGlueJxr.c:1874
```

Considering that the tests still pass, I think this is a safe solution
to this warning.

image/decode/segdec.c

@@ -961,8 +961,9 @@ Int DecodeMacroblockLowpass (CWMImageStrCodec * pSC, CCodingContext *pContext,
                         pCoeffs[k] += getBits (pIO, iModelBits);
                     }
                     else if (pCoeffs[k] < 0) {
-                        pCoeffs[k] <<= iModelBits;
-                        pCoeffs[k] -= getBits (pIO, iModelBits);
+                        // left-shift the positive value and set the negative sign back afterwards
+                        pCoeffs[k] = -(-pCoeffs[k] << iModelBits);
+                        pCoeffs[k] -= getBits(pIO, iModelBits);
                     }
 #endif // WIN32
                     else {