patch 8.1.0056: crash when using :hardcopy with illegal byte

Problem:    Crash when using :hardcopy with illegal byte.
Solution:   Check for string_convert() returning NULL. (Dominique Pelle)
diff --git a/src/hardcopy.c b/src/hardcopy.c
index 91f62c3..420ceb7 100644
--- a/src/hardcopy.c
+++ b/src/hardcopy.c
@@ -3372,8 +3372,9 @@
 }
 
     int
-mch_print_text_out(char_u *p, int len UNUSED)
+mch_print_text_out(char_u *textp, int len UNUSED)
 {
+    char_u	*p = textp;
     int		need_break;
     char_u	ch;
     char_u      ch_buff[8];
@@ -3508,8 +3509,15 @@
 
 #ifdef FEAT_MBYTE
     if (prt_do_conv)
+    {
 	/* Convert from multi-byte to 8-bit encoding */
 	tofree = p = string_convert(&prt_conv, p, &len);
+	if (p == NULL)
+	{
+	    p = (char_u *)"";
+	    len = 0;
+	}
+    }
 
     if (prt_out_mbyte)
     {