[vala/0.46] vala: Ensure non-empty argument list for "disconnect" before accessing it

[Rico Tzschichholz <ricotz src gnome org>]

commit 17c55d12326d490c58e44a50850c3064eabae4d7
Author: Princeton Ferro <princetonferro gmail com>
Date:   Fri Apr 17 02:45:12 2020 -0400

    vala: Ensure non-empty argument list for "disconnect" before accessing it
    
    Avoids an internal assertion on accessing an empty collection.

 tests/Makefile.am                                   |  1 +
 tests/semantic/signal-disconnect-empty-invalid.test | 10 ++++++++++
 vala/valamethodcall.vala                            |  3 +--
 3 files changed, 12 insertions(+), 2 deletions(-)
---
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 1d1739fb9..6925ebda3 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -869,6 +869,7 @@ TESTS = \
        semantic/signal-detail-invalid.test \
        semantic/signal-detail-non-string.test \
        semantic/struct-derived.test \
+       semantic/signal-disconnect-empty-invalid.test \
        semantic/signal-disconnect-invalid.test \
        semantic/signal-return-valist.test \
        semantic/struct-field-initializer.test \
diff --git a/tests/semantic/signal-disconnect-empty-invalid.test 
b/tests/semantic/signal-disconnect-empty-invalid.test
new file mode 100644
index 000000000..2830ed975
--- /dev/null
+++ b/tests/semantic/signal-disconnect-empty-invalid.test
@@ -0,0 +1,10 @@
+Invalid Code
+
+class Foo : Object {
+       public signal void bar ();
+}
+
+void main () {
+       var foo = new Foo ();
+       foo.bar.disconnect ();
+}
diff --git a/vala/valamethodcall.vala b/vala/valamethodcall.vala
index baa451e20..f71eb957c 100644
--- a/vala/valamethodcall.vala
+++ b/vala/valamethodcall.vala
@@ -541,8 +541,7 @@ public class Vala.MethodCall : Expression {
 
                        unowned Signal? sig = m.parent_symbol as Signal;
                        if (sig != null && m.name == "disconnect") {
-                               var param = get_argument_list ()[0];
-                               if (param is LambdaExpression) {
+                               if (!argument_list.is_empty && argument_list[0] is LambdaExpression) {
                                        error = true;
                                        Report.error (source_reference, "Cannot disconnect lambda expression 
from signal");
                                        return false;