Prevent ELIF or ELSE after an ELSE

Fixes #749

test/asm/skip-elif-condition.asm

@@ -0,0 +1,17 @@
+mac: MACRO
+	if (\1) < 10
+		println "small \1"
+	elif (\1) > 100
+		println "large \1"
+	elif (\1) / 0 == 42 ; only evaluated if the "large" condition was taken
+		println "division by zero!?"
+	elif syntax! error?
+		println "X_X"
+	else
+		println "unreachable"
+	endc
+ENDM
+
+	mac 2 + 2
+	mac STRLEN("abcdef")
+	mac 101