patch 8.2.2515: memory access error when truncating an empty message

Problem:    Memory access error when truncating an empty message.
Solution:   Check for an empty string. (Dominique Pellé, closes #7841)

src/message_test.c

@@ -49,6 +49,15 @@ test_trunc_string(void)
     char_u  *buf; /*allocated every time to find uninit errors */
     char_u  *s;
 
+    // Should not write anything to destination if buflen is 0.
+    trunc_string((char_u *)"", NULL, 1, 0);
+
+    // Truncating an empty string does nothing.
+    buf = alloc(1);
+    trunc_string((char_u *)"", buf, 1, 1);
+    assert(buf[0] == NUL);
+    vim_free(buf);
+
     // in place
     buf = alloc(40);
     STRCPY(buf, "text");