libmm-glib,common-helpers: make hexstr2bin() return a GError

This util method checks whether the input string is a valid hex
string, so make sure we return a GError on failure.

src/mm-charsets.c

@@ -169,7 +169,7 @@ mm_modem_charset_hex_to_utf8 (const gchar    *src,
     iconv_from = charset_iconv_from (charset);
     g_return_val_if_fail (iconv_from != NULL, FALSE);
 
-    unconverted = mm_utils_hexstr2bin (src, &unconverted_len);
+    unconverted = mm_utils_hexstr2bin (src, &unconverted_len, NULL);
     if (!unconverted)
         return NULL;