Fix macro and rept buffer overflows

Macro and rept buffers were not always being terminated with newlines
and/or were vulnerable to the final newline being escaped, allowing
buffer overflows to occur. Now, they are terminated with newlines using
the same mechanism as the file buffer.

test/asm/line-continuation-macro.asm

@@ -0,0 +1,7 @@
+m: MACRO
+ENDM
+
+m2: MACRO
+	m \ ENDM
+
+	m2

test/asm/line-continuation-rept.asm

@@ -0,0 +1,8 @@
+m: MACRO
+ENDM
+
+REPT 1
+	m ENDR
+
+REPT 1
+	m \ ENDR