patch 8.2.1335: CTRL-C in the GUI doesn't interrupt

Problem:    CTRL-C in the GUI doesn't interrupt. (Sergey Vlasov)
Solution:   Recognize "C" with CTRL modifier as CTRL-C. (issue #6565)
diff --git a/src/gui_photon.c b/src/gui_photon.c
index cdb5e15..12b0a3c 100644
--- a/src/gui_photon.c
+++ b/src/gui_photon.c
@@ -596,11 +596,17 @@
 	    string[ len++ ] = ch;
 	}
 
-	if (len == 1 && ((ch == Ctrl_C && ctrl_c_interrupts)
-							  || ch == intr_char))
+	// Check if the key interrupts.
 	{
-	    trash_input_buf();
-	    got_int = TRUE;
+	    int int_ch = check_for_interrupt(ch, modifiers);
+
+	    if (int_ch != NUL)
+	    {
+		ch = int_ch;
+		string[0] = ch;
+		len = 1;
+		trash_input_buf();
+	    }
 	}
 
 	if (len == 1 && string[0] == CSI)