gguf : opening an invalid file may cause a out of bounds access

[slaren]

It is not guaranteed that magic is NUL terminated, but it is printed as a string:

ggml/src/ggml.c

Lines 18025 to 18038 in cce2ac9

	char magic[4];
	// check the magic before making allocations
	{
	gguf_fread_el(file, &magic, sizeof(magic), &offset);
	for (uint32_t i = 0; i < sizeof(magic); i++) {
	if (magic[i] != GGUF_MAGIC[i]) {
	fprintf(stderr, "%s: invalid magic characters %s.\n", __func__, magic);
	fclose(file);
	return NULL;
	}
	}
	}