YANG parser BUGFIX reading escaped characters in quoted strings

The \t and \n sequence is actually stored as a single character so it
modifies the resulting and to store it, a dynamically allocated buffer
is needed.
diff --git a/src/parser_yang.c b/src/parser_yang.c
index 314cbd7..ca713b9 100644
--- a/src/parser_yang.c
+++ b/src/parser_yang.c
@@ -401,9 +401,11 @@
             switch (**data) {
             case 'n':
                 c = "\n";
+                need_buf = 1;
                 break;
             case 't':
                 c = "\t";
+                need_buf = 1;
                 break;
             case '\"':
                 c = *data;