swscale/aarch64: fix signed load in yuv2planeX_10

The horizontal scaler intermediate buffer holds signed 16-bit samples
that can be negative when scaling with filters that have negative lobes
(such as bicubic). The 16-pixel and 8-pixel NEON vector loops in
ff_yuv2planeX_10_neon treat src as signed int16_t via smlal, but the
scalar tail loop for remaining (dstW % 8) pixels used unsigned ldrh
before sxtw, interpreting negative intermediate values as large positive
values around 65536 and saturating output pixels.

Use ldrsh instead of ldrh so negative intermediate samples are properly
sign-extended in the scalar tail loop.

Signed-off-by: Marvin Scholz <epirat07@gmail.com>

libswscale/aarch64/output.S

@@ -165,7 +165,7 @@ function ff_yuv2planeX_10_neon, export=1
         ldr             x14, [x12], #8                  // Load src pointer
         ldrsh           w15, [x13], #2                  // Load filter coefficient
         add             x14, x14, x7, lsl #1            // Add pixel offset
-        ldrh            w16, [x14]
+        ldrsh           w16, [x14]
 
         sxtw            x16, w16
         sxtw            x15, w15