Do not evaluate an untaken ELIF's condition

Fixes #764

test/asm/elif-after-taken-if.asm

@@ -0,0 +1,23 @@
+if 1
+	println "taken if"
+elif 2 / 0 ; avoided fatal "Division by zero" error
+	println "untaken elif"
+elif 3 / 0 ; avoided fatal "Division by zero" error
+	println "untaken after untaken"
+endc
+
+if 0
+	println "untaken if"
+elif 1
+	println "taken elif"
+elif !@#$ ; avoided fatal syntax error
+	println "untaken elif"
+elif %^&* ; avoided fatal syntax error
+	println "untaken after untaken"
+endc
+
+if 0
+	println "untaken if"
+elif 1 / 0 ; fatal "Division by zero" error
+	println "unreached elif"
+endc

test/asm/elif-after-taken-if.err

@@ -0,0 +1,2 @@
+FATAL: elif-after-taken-if.asm(21):
+    Division by zero

test/asm/elif-after-taken-if.out

@@ -0,0 +1,2 @@
+taken if
+taken elif

test/asm/skip-elif-condition.asm

@@ -3,7 +3,7 @@ mac: MACRO
 		println "small \1"
 	elif (\1) > 100
 		println "large \1"
-	elif (\1) / 0 == 42 ; only evaluated if the "large" condition was taken
+	elif (\1) / 0 == 42 ; only evaluated if neither "small" nor "large" was taken
 		println "division by zero!?"
 	elif syntax! error?
 		println "X_X"
@@ -15,3 +15,4 @@ ENDM
 	mac 2 + 2
 	mac STRLEN("abcdef")
 	mac 101
+	mac 23

test/asm/skip-elif-condition.err

@@ -1,2 +1,2 @@
-FATAL: skip-elif-condition.asm(17) -> skip-elif-condition.asm::mac(6):
+FATAL: skip-elif-condition.asm(18) -> skip-elif-condition.asm::mac(6):
     Division by zero