patch 7.4.1779

Problem:    Using negative index in strcharpart(). (Yegappan Lakshmanan)
Solution:   Assume single byte when using a negative iindex.

src/eval.c

@@ -19774,7 +19774,12 @@ f_strcharpart(typval_T *argvars, typval_T *rettv)
 	    charlen = get_tv_number(&argvars[2]);
 	    while (charlen > 0 && nbyte + len < slen)
 	    {
-		len += mb_char2len(p[nbyte + len]);
+		int off = nbyte + len;
+
+		if (off < 0)
+		    len += 1;
+		else
+		    len += mb_char2len(p[off]);
 		--charlen;
 	    }
 	}

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1779,
 /**/
     1778,
 /**/