FIX: Memory corruption in append/part with char! argument

resolves: Oldes/Rebol-issues#2683

Author: Oldes

src/core/f-modify.c

@@ -165,6 +165,7 @@
 		SERIES_TAIL(src_ser) = Encode_UTF8_Char(STR_HEAD(src_ser), VAL_CHAR(src_val));
 		TERM_SERIES(src_ser);
 		if (SERIES_TAIL(src_ser) > 1) UTF8_SERIES(src_ser);
+		dst_len = SERIES_TAIL(src_ser);
 	}
 	else if (IS_BLOCK(src_val)) {
 		src_ser = Form_Tight_Block(src_val);

src/tests/units/series-test.r3

@@ -751,6 +751,9 @@ Rebol [
 	--test-- "APPEND string! char!"
 		--assert "a" = append "" #"a"
 		--assert "←" = append "" #"^(2190)" ; wide char
+		;@@ https://github.com/Oldes/Rebol-issues/issues/2683
+		--assert "a" = append/part "" #"a" 10
+		--assert "←" = append/part "" #"^(2190)" 10
 ===end-group===
 
 ===start-group=== "APPEND ref!"