Fix ICE caused by invalid spans for shrink_file
Fixesrust-lang/rust#148732
There are two issues in this function:
1. the original issue is caused by a typo error, which is fixed in the first commit
2. another different ice(Patch span `7..7` is beyond the end of buffer `0`) will be reported after fixing the first one, is caused by spans cross file boundaries due to macro expansion. It is fixed in the second commit.
r? `@nnethercote`
edited: also fixesrust-lang/rust#148684, added a new testcase for it in the last commit.