Fix compile warnings in dlmalloc

The origional code was using on odd reference to get to the first
real element in av_[].  The first two elements of the array are
not used for actual bins, but for house keeping.  If we are more
explicit about how use the first few elements we can get rid of the
warnings:

dlmalloc.c: In function 'malloc_extend_top':
dlmalloc.c:1971: warning: dereferencing type-punned pointer will break strict-aliasing rules
dlmalloc.c:1999: warning: dereferencing type-punned pointer will break strict-aliasing rules
dlmalloc.c:2029: warning: dereferencing type-punned pointer will break strict-aliasing rules
...

The logic of how this code came to be is:
	bin_at(0) = (char*)&(av_[2]) - 2*SIZE_SZ

SIZE_SZ is the size of pointer, and av_ is arry of pointers so:
	bin_at(0) = &(av_[0])

Going from there to bin_at(0)->fd or bin_at(0)->size should be straight forward.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>