Fix expansions that start from the end of another expansion (#839)

Do not free an expansion until its offset is *past* its size.
This means potentially freeing a nested stack of expansions
all at once.

Fixes #696

test/asm/test.sh

@@ -62,6 +62,7 @@ EOF
 	cat > quote\"file.err <<EOF
 warning: quote"file.asm(1): [-Wuser]
     quote"file.asm
+while expanding symbol "__FILE__"
 EOF
 fi