Fix crash on unary semantic / error

Author: boriel

tests/functional/unary_crash.bas

@@ -0,0 +1,3 @@
+
+a = -b(4)
+

zxb/zxbparser.py

@@ -190,6 +190,9 @@ def make_binary(lineno, operator, left, right, func=None, type_=None):
 def make_unary(lineno, operator, operand, func=None, type_=None):
     """ Wrapper: returns a Unary node
     """
+    if operand is None:  # syntax / semantic error
+        return None
+
     return symbols.UNARY.make_node(lineno, operator, operand, func, type_)