snapuserd: Fix PrepareReadAhead

When checking the instead of current, and when checking the position, we could underflow,
which would get caught by UBSAN

Bug: 177104308
Test: Map cow file with xor op or copy op at position 0
Change-Id: I4fba1982419e08b58759ec893e238175d245dbfc

parent bb0a7642

fs_mgr/libsnapshot/snapuserd/snapuserd_readahead.cpp

+1 −1

Original line number	Diff line number	Diff line
		@@ -223,7 +223,7 @@ void ReadAheadThread::PrepareReadAhead(uint64_t* source_offset, int* pending_ops
		if (op->type == kCowCopyOp) {
		next_offset *= BLOCK_SZ;
		}
		if (next_offset != (source_offset - nr_consecutive BLOCK_SZ)) {
		if (next_offset + nr_consecutive * BLOCK_SZ != *source_offset) {
		break;
		}
		nr_consecutive += 1;