Fix failing assertion with backslash at EOF in macro arguments (#1634)

`Expansion::advance()` can increase its offset beyond the size,
so I don't think this assumption was valid in the first place;
`BufferedContent::advance()` should be able to as well.

test/asm/command-line-symbols.asm

@@ -0,0 +1,3 @@
+assert !strcmp("{FOO}", "hello")
+assert !strcmp("{DEFINED}", "1")
+def FOO equ 42