charset: fix mm_charset_get_encoded_len

The while loop in mm_charset_get_encoded_len() iterates through each
valid UTF-8 encoded character in the given NULL-terminated UTF-8 string.
It uses g_utf8_find_next_char() to find the position of the next
character. In case, g_utf8_find_next_char() returns NULL, it tries to
find the end (i.e. the NULL character) of the string.

This patch fixes the following issues in the while loop:

1. The loop uses both 'next' and 'end' to track the position of the next
   character in the string.

   When g_utf8_find_next_char() returns a non-NULL value, 'next' is
   essentially the same as 'end'.

   When g_utf8_find_next_char() returns NULL, 'next' remains NULL while
   'end' is adjusted to track the end of the string (but is done
   incorrectly as described in #2). After the 'p = next' assignment, the
   'while (*p)' check results in a NULL dereference. 'p' should thus be
   set to 'end' instead of 'next'.

   'next' is thus redundant and can be removed.

2. When g_utf8_find_next_char() returns NULL and 'end' is adjusted to
   track the end of string, the 'while (*end++)' loop stops when finding
   the NULL character, but 'end' is advanced past the NULL character.
   After the 'p = end' assignment, the 'while (*p)' check results in a
   dereference of out-of-bounds pointer.

   'while (*++end)' should be used instead given that 'p' doesn't point
   to a NULL character when 'end = p' happens. 'end' will be updated to
   point to the NULL character. After the 'p = end' assignment (fixed in
   #1), the 'while (*p)' check will properly stop the loop.

src/mm-charsets.c

@@ -595,7 +595,7 @@ mm_charset_get_encoded_len (const char *utf8,
                             MMModemCharset charset,
                             guint *out_unsupported)
 {
-    const char *p = utf8, *next;
+    const char *p = utf8;
     guint len = 0, unsupported = 0;
     SubsetEntry *e;
 
@@ -618,17 +618,17 @@ mm_charset_get_encoded_len (const char *utf8,
 
         c = g_utf8_get_char_validated (p, -1);
         g_return_val_if_fail (c != (gunichar) -1, 0);
-        end = next = g_utf8_find_next_char (p, NULL);
+        end = g_utf8_find_next_char (p, NULL);
         if (end == NULL) {
-            /* Find the end... */
+            /* Find the string terminating NULL */
             end = p;
-            while (*end++);
+            while (*++end);
         }
 
         if (!e->func (c, p, (end - p), &clen))
             unsupported++;
         len += clen;
-        p = next;
+        p = end;
     }
 
     if (out_unsupported)