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/macro-arg-escape-chars.asm

@@ -0,0 +1,5 @@
+MACRO mac
+	DEF s EQUS "\#"
+	println "{#s:s}"
+ENDM
+mac \\\"\t\r\0\n\{\}\,\(\)\w\