allow for 2Gb+ memory allocations on 64-bit Win32 debug builds

allow for 2Gb+ memory allocations on 64-bit Win32 debug builds

long is 32-bit for the 64-bit Win32 memory model, so use the more
portable SSize_t.

Before the change:

C:\Users\tony\dev\perl\git\perl>perl -e "open my $f, 'dir |' or die; $x = ''; re
ad($f, $x, 4, 0x80000000)"
panic: realloc at -e line 1.

and after:

C:\Users\tony\dev\perl\git\perl>perl -e "open my $f, 'dir |' or die; $x = ''; re
ad($f, $x, 4, 0x80000000)"

C:\Users\tony\dev\perl\git\perl>git add util.c