Infinite loop with nested sections and globals

Using Catch 1.2.1, I've found an issue which causes infinite loops. I've whittled down my code to a minimum reproducible test case:

``` c++
#define CATCH_CONFIG_MAIN
#include "catch.hpp"

int a = 0;

TEST_CASE("") {
SECTION("") {
SECTION("") {
REQUIRE(a==0);

SECTION("") {
a=1;

SECTION("") {
}
SECTION("") {
}
}
}
}
}
```

I compiled this with `gcc version 4.9.3 (Debian 4.9.3-3)`

This loops forever with output like this:

```
a.out is a Catch v1.2.1 host application.
Run with -? for options

-------------------------------------------------------------------------------
Anonymous test case 1

-------------------------------------------------------------------------------
test_case.cpp:6
...............................................................................

test_case.cpp:9: FAILED:
REQUIRE( a==0 )
with expansion:
1 == 0

-------------------------------------------------------------------------------
Anonymous test case 1

-------------------------------------------------------------------------------
test_case.cpp:6
...............................................................................

test_case.cpp:9: FAILED:
REQUIRE( a==0 )
with expansion:
1 == 0
```