Don't consider difference of invalid labels constant

If a label is defined outside of a section, avoid trying to obtain its
value.

test/asm/use-label-outside-section.asm

@@ -0,0 +1,3 @@
+lab:
+	PRINTV lab-lab
+	PRINTT "\n"

test/asm/use-label-outside-section.err

@@ -0,0 +1,5 @@
+ERROR: use-label-outside-section.asm(1):
+    Label "lab" created outside of a SECTION
+ERROR: use-label-outside-section.asm(2):
+    Expected constant expression: 'lab' is not constant at assembly time
+error: Assembly aborted (2 errors)!

test/asm/use-label-outside-section.out

@@ -0,0 +1 @@
+$0