Fix segmentation fault on strftime/1 and strflocaltime/1

[itchyny]

When we use a large format string to strftime/1 and strflocaltime/1,
we get segmentation fault. This happens because the result buffer is
allocated on the stack using alloca function, and strftime function
writes out of the space when the size is not enough to format time.
I fixed the segmentation fault issue by allocating the result buffer on
the heap using malloc function, because alloca function does not
provide a way to detect insufficient space.