null reader refs used by parsers after use

Signed-off-by: James Cherry <cherry@parallaxsw.com>

liberty/LibertyExpr.cc

@@ -39,7 +39,9 @@ parseFuncExpr(const char *func,
     LibExprParser parser(func, cell, error_msg, report);
     libexpr_parser = &parser;
     LibertyExprParse_parse();
-    return parser.result();
+    FuncExpr *expr = parser.result();
+    libexpr_parser = nullptr;
+    return expr;
   }
   else
     return nullptr;

parasitics/SpefReader.cc

@@ -72,6 +72,7 @@ readSpefFile(const char *filename,
     // yyparse returns 0 on success.
     success = (::SpefParse_parse() == 0);
     gzclose(stream);
+    spef_reader = nullptr;
   }
   else
     throw FileNotReadable(filename);