swscale/x86/swscale: Fix undefined left shifts of negative numbers

This affected many FATE-tests: The number of failing tests went down
from 663 to 344. (Both numbers exclude tests that failed because of
unaligned accesses in code that is inside #if HAVE_FAST_UNALIGNED.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 736c7c20e7)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libswscale/x86/swscale.c

@@ -160,7 +160,7 @@ void ff_updateMMXDitherTables(SwsContext *c, int dstY, int lumBufIndex, int chrB
                 *(const void**)&lumMmxFilter[s*i+APCK_PTR2/4  ]= lumSrcPtr[i+(vLumFilterSize>1)];
                 lumMmxFilter[s*i+APCK_COEF/4  ]=
                 lumMmxFilter[s*i+APCK_COEF/4+1]= vLumFilter[dstY*vLumFilterSize + i    ]
-                + (vLumFilterSize>1 ? vLumFilter[dstY*vLumFilterSize + i + 1]<<16 : 0);
+                    + (vLumFilterSize>1 ? vLumFilter[dstY*vLumFilterSize + i + 1] * (1 << 16) : 0);
                 if (CONFIG_SWSCALE_ALPHA && hasAlpha) {
                     *(const void**)&alpMmxFilter[s*i              ]= alpSrcPtr[i  ];
                     *(const void**)&alpMmxFilter[s*i+APCK_PTR2/4  ]= alpSrcPtr[i+(vLumFilterSize>1)];
@@ -173,7 +173,7 @@ void ff_updateMMXDitherTables(SwsContext *c, int dstY, int lumBufIndex, int chrB
                 *(const void**)&chrMmxFilter[s*i+APCK_PTR2/4  ]= chrUSrcPtr[i+(vChrFilterSize>1)];
                 chrMmxFilter[s*i+APCK_COEF/4  ]=
                 chrMmxFilter[s*i+APCK_COEF/4+1]= vChrFilter[chrDstY*vChrFilterSize + i    ]
-                + (vChrFilterSize>1 ? vChrFilter[chrDstY*vChrFilterSize + i + 1]<<16 : 0);
+                    + (vChrFilterSize>1 ? vChrFilter[chrDstY*vChrFilterSize + i + 1] * (1 << 16) : 0);
             }
         } else {
             for (i=0; i<vLumFilterSize; i++) {