scformat: use vmalloc instead of vmresize when processing '['

scformat: use vmalloc instead of vmresize when processing '['

Though it is hard to see, the previous code bottomed out in a call to vmresize.
After allocating, it zeroes the allocated region. So vmresize (itself calling
realloc) signals an unnecessary constraint to the allocator that previous data
in this memory must be preserved.

To simplify this, this change converts the sequence to a vmfree of the prior
memory and then a new vmalloc, more clearly divorcing the new buffer from the
old one.