If realloc fails, do not reduce block size

[radarhere]

Within this code, it is possible for realloc to fail.

Pillow/src/libImaging/Storage.c

Lines 385 to 389 in 2d18dac

	// Reduce block size
	if (block.size > arena->block_size) {
	block.size = arena->block_size;
	block.ptr = realloc(block.ptr, arena->block_size);
	}

This doesn't have to trigger an error though. We can simply avoid reducing the block size.