train : fix #4227 (double free in examples/train-text-from-scratch/train-text-from-scratch.cpp) (#4351)

On commit b1108 (44c117f4) xaedes added

    ggml_allocr * alloc = NULL;

    ... (many lines in between)

    if (alloc) {
        ggml_allocr_free(alloc);
    }

Which is correct, but it's easy to lose context after many lines in between.

On commit b1287 (0e76a899) xaedes made a big change. From here on, alloc is freed eagerly.

    alloc = ggml_allocr_new(...)
    ... (short lines of code)
    ggml_allocr_free(alloc)

This happens a few times, but alloc is never set to NULL, and many lines below,
we still have

    if (alloc) {
        ggml_allocr_free(alloc);
    }

which causes a double-free.