patch 8.2.3693: Coverity warns for possibly using a NULL pointer

Problem:    Coverity warns for possibly using a NULL pointer.
Solution:   Check for NULL and give an error.
diff --git a/src/errors.h b/src/errors.h
index 3c76bb7..5cc98e7 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -355,7 +355,8 @@
 	INIT(= N_("E1084: Cannot delete Vim9 script function %s"));
 EXTERN char e_not_callable_type_str[]
 	INIT(= N_("E1085: Not a callable type: %s"));
-// E1086 unused
+EXTERN char e_function_reference_invalid[]
+	INIT(= N_("E1086: Function reference invalid"));
 EXTERN char e_cannot_use_index_when_declaring_variable[]
 	INIT(= N_("E1087: Cannot use an index when declaring a variable"));
 // E1088 unused
diff --git a/src/version.c b/src/version.c
index 90021be..b7fe97b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3693,
+/**/
     3692,
 /**/
     3691,
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 7704635..de4de16 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -3189,6 +3189,12 @@
 		    {
 			ufunc = find_func(funcref->fr_func_name, FALSE, NULL);
 		    }
+		    if (ufunc == NULL)
+		    {
+			SOURCING_LNUM = iptr->isn_lnum;
+			emsg(_(e_function_reference_invalid));
+			goto theend;
+		    }
 		    if (fill_partial_and_closure(pt, ufunc, ectx) == FAIL)
 			goto theend;
 		    tv = STACK_TV_BOT(0);