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/string-literal-macro-arg.asm

@@ -0,0 +1,11 @@
+MACRO mac
+	println "\1"
+	println \1
+ENDM
+mac "hello \\\"\t\r\0\n\ ; comment
+ \wor\
+ld"
+mac """goodbye
+cruel\	; comment
+\nworld"""
+mac "\