Fix thread-safety issue in debug.c

Closes #203
diff --git a/include/polarssl/debug.h b/include/polarssl/debug.h
index fcf1490..2dd89a2 100644
--- a/include/polarssl/debug.h
+++ b/include/polarssl/debug.h
@@ -57,7 +57,7 @@
 
 
 #define SSL_DEBUG_MSG( level, args )                    \
-    debug_print_msg( ssl, level, __FILE__, __LINE__, debug_fmt args );
+    debug_print_msg_free( ssl, level, __FILE__, __LINE__, debug_fmt args );
 
 #define SSL_DEBUG_RET( level, text, ret )                \
     debug_print_ret( ssl, level, __FILE__, __LINE__, text, ret );
@@ -115,6 +115,9 @@
 
 char *debug_fmt( const char *format, ... );
 
+void debug_print_msg_free( const ssl_context *ssl, int level,
+                           const char *file, int line, char *text );
+
 void debug_print_msg( const ssl_context *ssl, int level,
                       const char *file, int line, const char *text );