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.err

@@ -0,0 +1,9 @@
+error: string-literal-macro-arg.asm(6):
+    Illegal character escape 'w'
+error: string-literal-macro-arg.asm(11):
+    Illegal character escape at end of input
+error: string-literal-macro-arg.asm(11):
+    Unterminated string
+error: string-literal-macro-arg.asm(11) -> string-literal-macro-arg.asm::mac(4):
+    Unterminated string
+error: Assembly aborted (4 errors)!