resolveSubtable: Fix buffer overflow parsing a malformed table

The subtable's name can theoretically be up to MAXSTRING characters long.
The base name is then copied into a buffer, and the subtable's name is
appended, so we should allocate more than MAXSTRING bytes for the buffer.

Fixes CVE-2017-13739, CVE-2017-13740, and CVE-2017-13742.

liblouis/compileTranslationTable.c

@@ -4980,7 +4980,7 @@ resolveSubtable (const char *table, const char *base, const char *searchPath)
 
   if (table == NULL || table[0] == '\0')
     return NULL;
-  tableFile = (char *) malloc (MAXSTRING * sizeof(char));
+  tableFile = (char *) malloc (MAXSTRING * sizeof(char) * 2);
 
   //
   // First try to resolve against base