proresdec: Fix end condition

Fixes out of array writes
No FFmpeg release is affected by this

This also fixes some artifacts

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/proresdec2.c

@@ -458,7 +458,7 @@ static void unpack_alpha(GetBitContext *gb, uint16_t *dst, int num_coeffs,
             } else {
                 dst[idx++] = (alpha_val << 2) | (alpha_val >> 6);
             }
-            if (idx == num_coeffs - 1)
+            if (idx >= num_coeffs)
                 break;
         } while (get_bits_left(gb)>0 && get_bits1(gb));
         val = get_bits(gb, 4);

libavcodec/proresdec_lgpl.c

@@ -496,7 +496,7 @@ static void unpack_alpha(GetBitContext *gb, uint16_t *dst, int num_coeffs,
                 dst[idx++] = alpha_val >> 6;
             else
                 dst[idx++] = (alpha_val << 2) | (alpha_val >> 6);
-            if (idx == num_coeffs - 1)
+            if (idx >= num_coeffs)
                 break;
         } while (get_bits1(gb));
         val = get_bits(gb, 4);

tests/ref/fate/prores-transparency

 #tb 0: 1/25
 #tb 1: 1/48000
-0,          0,          0,        1, 16588800, 0x87f98f3c
+0,          0,          0,        1, 16588800, 0x20778f5e
 1,          0,          0,     1024,     4096, 0x00000000
 1,       1024,       1024,      896,     3584, 0x00000000