Remove extra entry in error stack on macro not defined (#394)

While working on #392, I noticed that the macro-@ test (as well
as the line-continuation test, but for that one see #393)
printed an additional '@(-1)' entry which doesn't make sense.

test/asm/macro-@.out

@@ -1,2 +1,2 @@
-ERROR: [email protected](1) -> @(-1):
+ERROR: [email protected](1):
     Macro '@' not defined

test/asm/macro-@.out.pipe

@@ -1,2 +1,2 @@
-ERROR: -(1) -> @(-1):
+ERROR: -(1):
     Macro '@' not defined