Merged in bugfix/parser_crash (pull request boriel-basic#217)

Bugfix/parser crash

Approved-by: Jose Rodriguez <boriel@gmail.com>

tests/functional/callable_err.bas

@@ -0,0 +1,3 @@
+IF m$(s(1)-vm,s(2)-hm)="\c" THEN REM
+
+

tests/functional/substr_err.bas

@@ -0,0 +1,2 @@
+IF k$<>s$ THEN REM
+IF m$(s(1),s(2))="\b" THEN LET m$(s(1),s(2))="\c"

zxbparser.py

@@ -1166,10 +1166,17 @@ def p_substr_assignment(p):
     if entry.class_ == CLASS.unknown:
         entry.class_ = CLASS.var
 
-    assert entry.class_ == CLASS.var and entry.type_ == TYPE.string
+    if entry.class_ != CLASS.var:
+        api.errmsg.syntax_error_cannot_assign_not_a_var(p.lineno(2), p[2])
+        return
+
+    if entry.type_ != TYPE.string:
+        api.errmsg.syntax_error_expected_string(p.lineno(2), entry.type_)
+        return
 
     if p[5].type_ != TYPE.string:
         api.errmsg.syntax_error_expected_string(p.lineno(4), p[5].type_)
+        return
 
     if len(p[3]) > 1:
         syntax_error(p.lineno(2), "Accessing string with too many indexes. Expected only one.")
@@ -2583,6 +2590,10 @@ def p_expr_funccall(p):
 def p_idcall_expr(p):
     """ func_call : ID arg_list %prec UMINUS
     """  # This can be a function call or a string index
+    if p[2] is None:
+        p[0] = None
+        return
+
     p[0] = make_call(p[1], p.lineno(1), p[2])
     if p[0] is None:
         return