More calloc conversions.

src/backend.cmod

src/modules/Image/colortable.c

src/modules/Image/encodings/_xpm.c

src/modules/Image/encodings/wbf.c

src/modules/Image/image.c

src/modules/Math/matrix_code.h

src/modules/_WhiteFish/blob.c

src/modules/_WhiteFish/buffer.c

src/modules/_WhiteFish/whitefish.c

src/pike_types.c

src/post_modules/Shuffler/a_source_pikestring.c

src/post_modules/Shuffler/a_source_system_memory.c

src/post_modules/Shuffler/b_source_normal_file.c

src/post_modules/Shuffler/c_source_stream.c

src/post_modules/Shuffler/d_source_pikestream.c

src/post_modules/Shuffler/e_source_block_pikestream.c

src/stralloc.c