Don't fail with assert() in string functions if both strings are NULL.

wstrconcat and wstrappend already support situations in which first OR
second string are NULL, but not if BOTH are NULL.  Fixed that.

WINGs/string.c

@@ -208,10 +208,12 @@ char *wstrconcat(char *str1, char *str2)
 	char *str;
 	size_t slen;
 
-	if (!str1)
+	if (!str1 && str2)
 		return wstrdup(str2);
-	else if (!str2)
+	else if (str1 && !str2)
 		return wstrdup(str1);
+	else if (!str1 && !str2)
+		return NULL;
 
 	slen = strlen(str1) + strlen(str2) + 1;
 	str = wmalloc(slen);
@@ -228,10 +230,10 @@ char *wstrappend(char *dst, char *src)
 {
 	size_t slen;
 
-	if (!dst)
-		return wstrdup(src);
-	else if (!src || *src == 0)
+	if (!src || *src == 0)
 		return dst;
+	else if (!dst)
+		return wstrdup(src);
 
 	slen = strlen(dst) + strlen(src) + 1;
 	dst = wrealloc(dst, slen);