Fix potential heap corruption on Windows

If len is large enough, when cast to an int it will be negative and then the
test if( len > MAX_PATH - 3 ) will not behave as expected.

Ref: IOTSSL-518

backport of 261faed725e29bd34abbf1f95df5fab43791f40c
diff --git a/ChangeLog b/ChangeLog
index ef4535f..6529b66 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,11 +1,18 @@
 PolarSSL ChangeLog
 
+= Version 1.2.18 released 2015-10-xx
+
+Security
+   * Fix potential heap corruption on Windows when
+     x509_crt_parse_path() is passed a path longer than 2GB. Cannot be
+     triggered remotely. Found by Guido Vranken, Interlworks.
+
 = Version 1.2.17 released 2015-10-06
 
 Security
-   * Fix for CVE-2015-5291. Possible heap buffer overflow in SSL if a very long
-     hostname is used. Can be trigerred remotely if you accept hostnames from
-     untrusted parties. Found by Guido Vranken, Intelworks.
+   * Fix for CVE-2015-5291 to prevent heap corruption due to buffer
+     overflow of the hostname or session ticket. Found by Guido Vranken,
+     Intelworks.
    * Fix stack buffer overflow in pkcs12 decryption (used by
      mbedtls_pk_parse_key(file)() when the password is > 129 bytes. Found by
      Guido Vranken, Intelworks. Not triggerable remotely.
diff --git a/library/x509parse.c b/library/x509parse.c
index c98145b..11e7841 100644
--- a/library/x509parse.c
+++ b/library/x509parse.c
@@ -1932,7 +1932,7 @@
     WCHAR szDir[MAX_PATH];
     char filename[MAX_PATH];
 	char *p;
-    int len = strlen( path );
+    size_t len = strlen( path );
 
 	WIN32_FIND_DATAW file_data;
     HANDLE hFind;
@@ -1947,7 +1947,7 @@
 	p = filename + len;
     filename[len++] = '*';
 
-	w_ret = MultiByteToWideChar( CP_ACP, 0, filename, len, szDir, MAX_PATH - 3 );
+	w_ret = MultiByteToWideChar( CP_ACP, 0, filename, (int) len, szDir, MAX_PATH - 3 );
     if( w_ret == 0 )
         return( POLARSSL_ERR_X509_INVALID_INPUT );
 
@@ -1965,7 +1965,7 @@
 
 		w_ret = WideCharToMultiByte( CP_ACP, 0, file_data.cFileName,
 									 lstrlenW(file_data.cFileName),
-									 p, len - 1,
+									 p, (int) len - 1,
 									 NULL, NULL );
         if( w_ret == 0 )
             return( POLARSSL_ERR_X509_FILE_IO_ERROR );