Fix usage problems with iconv

iconv_close() wasn't always being called.
iconv's buffer wasn't being reset when there was nothing to convert

Author: jkbzh

Changelog

@@ -4,6 +4,10 @@ Version Changes for Hypermail
 HYPERMAIL VERSION 2.4.0:
 ============================
 
+2018-11-04 Bill Shannon
+	* string.c
+	i18n_convstring(): fix usage problems with iconv
+
 2018-10-12 Jose Kahan
 	* setup.c
 	PreConfig(): removed a 64-bit warning

src/string.c

@@ -287,7 +287,6 @@ char *i18n_convstring(char *string, char *fromcharset, char *tocharset, size_t *
   }
   errno=0;
   ret=iconv(iconvfd, &string, &strleft, &convbuf, &bufleft);
-  iconv_close(iconvfd);
 
   if (ret==(size_t)-1){
     error = 1;
@@ -315,8 +314,11 @@ char *i18n_convstring(char *string, char *fromcharset, char *tocharset, size_t *
       break;
     }
   } else {
+    /* return to initial state */
+    iconv(iconvfd, NULL, NULL, &convbuf, &bufleft);
     error = 0;
   }
+  iconv_close(iconvfd);
 
   if (error) {
     origconvbuf[origlen]=0x0;