Fix interblock smoothing with narrow prog. JPEGs

Due to an oversight, the assignment of DC05, DC10, DC15, DC20, and DC25
(the right edge coefficients in the 5x5 interblock smoothing window) in
decompress_smooth_data() was incorrect for images exactly two MCU blocks
wide.  For such images, DC04, DC09, DC14, DC19, and DC24 were assigned
values based on the last MCU column, but DC05, DC10, DC15, DC20, and
DC25 were assigned values based on the first MCU column (because
block_num + 1 was never less than last_block_column.)  This commit
modifies jdcoefct.c so that, for images at least two MCU blocks wide,
DC05, DC10, DC15, DC20, and DC25 are assigned the same values as DC04,
DC09, DC14, DC19, and DC24 (respectively.)  DC05, DC10, DC15, DC20, and
DC25 are then immediately overwritten for images more than two MCU
blocks wide.

Since this issue was minor and not likely obvious to an end user, the
fix is undocumented.

Fixes #700

Author: dcommander

jdcoefct.c

@@ -583,11 +583,11 @@ decompress_smooth_data(j_decompress_ptr cinfo, JSAMPIMAGE output_buf)
         /* Update DC values */
         if (block_num == cinfo->master->first_MCU_col[ci] &&
             block_num < last_block_column) {
-          DC04 = (int)prev_prev_block_row[1][0];
-          DC09 = (int)prev_block_row[1][0];
-          DC14 = (int)buffer_ptr[1][0];
-          DC19 = (int)next_block_row[1][0];
-          DC24 = (int)next_next_block_row[1][0];
+          DC04 = DC05 = (int)prev_prev_block_row[1][0];
+          DC09 = DC10 = (int)prev_block_row[1][0];
+          DC14 = DC15 = (int)buffer_ptr[1][0];
+          DC19 = DC20 = (int)next_block_row[1][0];
+          DC24 = DC25 = (int)next_next_block_row[1][0];
         }
         if (block_num + 1 < last_block_column) {
           DC05 = (int)prev_prev_block_row[2][0];