dict.c: use av_mallocz instead of av_realloc

dict.c: use av_mallocz instead of av_realloc

Memory passed to av_realloc must come from malloc,
calloc or realloc, and not e.g. memalign. realloc(3):

The realloc() function changes the size of the memory block pointed to
by ptr to size bytes. (...) Unless ptr is NULL, it must have been
returned by an earlier call to malloc(), calloc() or realloc().

The issue has been found by debugallocation, a part of google-perftools:
http://code.google.com/p/gperftools/ .

This makes fate pass when using LD_PRELOAD-ed debugallocation.

See also earlier discussion
http://ffmpeg.org/pipermail/ffmpeg-devel/2013-January/137234.html

Signed-off-by: Paweł Hajdan, Jr <phajdan@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>