master: Build failure in malloc with GCC 7

[Florian Weimer]

This is from a recent build in Fedora rawhide:

malloc.c:1590:50: error: array subscript is above array bounds
[-Werror=array-bounds]
 #define fastbin(ar_ptr, idx) ((ar_ptr)->fastbinsY[idx])
                              ~~~~~~~~~~~~~~~~~~~~^~~~~~
malloc.c:3572:26: note: in expansion of macro 'fastbin'
       mfastbinptr *fb = &fastbin (av, idx);
                          ^~~~~~~

Does anyone else see this?  The code is not obviously wrong; someone
needs to look at GIMPLE dumps to figure out what is going on here.

Thanks,
Florian