Can produce broken DjVu files with --fg-colors=web

[jwilk]

If no background is detected, pdf2djvu 0.7.1 produces broken DjVu files:

$ gm convert -size 100x100 gradient:snow-navy test.pdf

$ pdf2djvu test.pdf -o test.djvu --fg-colors web
test.pdf:
- page #1 -> #1
0.085 bits/pixel; 1.439:1, 30.51% saved, 2645 bytes in, 1838 bytes out

$ djvudump test.djvu
  FORM:DJVM [1826] 
    DIRM [51]         Document directory (bundled, 2 files 1 pages)
    FORM:DJVM [1153] {p0001.djvu} [P1]
      DIRM [38]         Document directory (bundled, 2 files 2 pages)
      INCL [15]         Indirection chunk --> {shared_anno.iff}
      FORM:DJVU [34] 
        INFO [10]         DjVu 417x417, v24, 300 dpi, gamma=2.2
        Sjbz [4]          JB2 bilevel data
      FORM:DJVU [1029] 
        INFO [10]         DjVu 417x417, v24, 300 dpi, gamma=2.2
        Sjbz [155]        JB2 bilevel data
        FGbz [683]        JB2 colors data, v0, 216 colors
        BG44 [76]         IW4 data #1, 72 slices, v1.2 (color), 139x139
        BG44 [16]         IW4 data #2, 11 slices
        BG44 [14]         IW4 data #3, 10 slices
        BG44 [13]         IW4 data #4, 10 slices
    FORM:DJVI [592] {shared_anno.iff} [S]
      ANTz [580]        Page annotation (hyperlinks, etc.)

$ ddjvu test.djvu > /dev/null
ddjvu: Cannot decode page 1.

[jwilk]

Fixed in 72be13a3bc33817030125301fa34df3ddef63a10.

[jwilk]

Fixed in 0.7.2.