swscale/x86/rgb2rgb_template: Fix planar2x() for short width

Fixes: 451b3e0cf956c0bd2f27ed753ac24050/asan_heap-oob_2873c01_3231_7ed10a9464d15f0d57277f5917c566a8.AVI

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/x86/rgb2rgb_template.c

@@ -1434,7 +1434,9 @@ static inline void RENAME(planar2x)(const uint8_t *src, uint8_t *dst, int srcWid
     dst+= dstStride;
 
     for (y=1; y<srcHeight; y++) {
-        const x86_reg mmxSize= srcWidth&~15;
+        x86_reg mmxSize= srcWidth&~15;
+
+        if (mmxSize) {
         __asm__ volatile(
             "mov           %4, %%"REG_a"            \n\t"
             "movq        "MANGLE(mmx_ff)", %%mm0    \n\t"
@@ -1481,6 +1483,11 @@ static inline void RENAME(planar2x)(const uint8_t *src, uint8_t *dst, int srcWid
                NAMED_CONSTRAINTS_ADD(mmx_ff)
             : "%"REG_a
         );
+        } else {
+            mmxSize = 1;
+            dst[0]         = (src[0] * 3 + src[srcStride]) >> 2;
+            dst[dstStride] = (src[0] + 3 * src[srcStride]) >> 2;
+        }
 
         for (x=mmxSize-1; x<srcWidth-1; x++) {
             dst[2*x          +1]= (3*src[x+0] +   src[x+srcStride+1])>>2;