patch 8.0.0243: tolower() does not work if the byte count changes

Problem:    When making a character lower case with tolower() changes the byte
            cound, it is not made lower case.
Solution:   Add strlow_save(). (Dominique Pelle, closes #1406)

src/misc2.c

@@ -1602,7 +1602,10 @@ strup_save(char_u *orig)
 		{
 		    s = alloc((unsigned)STRLEN(res) + 1 + newl - l);
 		    if (s == NULL)
-			break;
+		    {
+			vim_free(res);
+			return NULL;
+		    }
 		    mch_memmove(s, res, p - res);
 		    STRCPY(s + (p - res) + newl, p + l);
 		    p = s + (p - res);
@@ -1625,6 +1628,69 @@ strup_save(char_u *orig)
 
     return res;
 }
+
+/*
+ * Make string "s" all lower-case and return it in allocated memory.
+ * Handles multi-byte characters as well as possible.
+ * Returns NULL when out of memory.
+ */
+    char_u *
+strlow_save(char_u *orig)
+{
+    char_u	*p;
+    char_u	*res;
+
+    res = p = vim_strsave(orig);
+
+    if (res != NULL)
+	while (*p != NUL)
+	{
+# ifdef FEAT_MBYTE
+	    int		l;
+
+	    if (enc_utf8)
+	    {
+		int	c, lc;
+		int	newl;
+		char_u	*s;
+
+		c = utf_ptr2char(p);
+		lc = utf_tolower(c);
+
+		/* Reallocate string when byte count changes.  This is rare,
+		 * thus it's OK to do another malloc()/free(). */
+		l = utf_ptr2len(p);
+		newl = utf_char2len(lc);
+		if (newl != l)
+		{
+		    s = alloc((unsigned)STRLEN(res) + 1 + newl - l);
+		    if (s == NULL)
+		    {
+			vim_free(res);
+			return NULL;
+		    }
+		    mch_memmove(s, res, p - res);
+		    STRCPY(s + (p - res) + newl, p + l);
+		    p = s + (p - res);
+		    vim_free(res);
+		    res = s;
+		}
+
+		utf_char2bytes(lc, p);
+		p += newl;
+	    }
+	    else if (has_mbyte && (l = (*mb_ptr2len)(p)) > 1)
+		p += l;		/* skip multi-byte character */
+	    else
+# endif
+	    {
+		*p = TOLOWER_LOC(*p); /* note that tolower() can be a macro */
+		p++;
+	    }
+	}
+
+    return res;
+}
 #endif
 
 /*