Fix failing assertion with backslash at EOF in macro arguments

[Rangi42]

This was going to just increase test coverage, but it turns out that piping one of the test cases so it's read from stdin causes an assertion failure! 🙃 ...Which is manifesting as an infinite loop on some systems (Windows, Cygwin, and statically-linked macOS).

[Rangi42]

First, this case (reduced from macro-arg-escape-chars.asm):

MACRO mac
ENDM
; no newline, backslash is last char
mac \

was causing assume(size > 0); to fail in BufferedContent::advance().

Based on comparing to Expansion::advance(), I don't think the assumption was valid.

Fixing that (by doing if (size > 0) { size--; } instead of assuming it and always decrementing) then caused/uncovered this case (reduced from string-literal-macro-arg.asm):

MACRO mac
	println "\1"
	println \1
ENDM
; no newline, backslash is last char
mac "\

to print different output on Windows and MinGW than on all the other platforms.

I fixed that one in #1635 and rebased this to it.