zero-terminate imap literals

now that we properly support literals for strings, we must expect that
the consumer code will use them as strings.

amends fc77feacc.

discovered by Armands Liepins <armandsl@gmail.com>

REFMAIL: CAF_KswXoxdm7KXnWW4b_1odf=XsE4qRqRN4AsecwcPF1d+dSTA@mail.gmail.com

src/drv_imap.c

@@ -687,7 +687,8 @@ parse_imap_list( imap_store_t *ctx, char **sp, parse_list_state_t *sts )
 			if (*s != '}' || *++s)
 				goto bail;
 
-			s = cur->val = nfmalloc( cur->len );
+			s = cur->val = nfmalloc( cur->len + 1 );
+			s[cur->len] = 0;
 
 		  getbytes:
 			bytes -= socket_read( &ctx->conn, s, bytes );