OverlappingFieldsCanBeMergedRule stack-overflows on cyclic fragment spreads

Validating a document that contains a cyclic fragment spread crashes the process with a stack overflow.

## Reproducer

```graphql
fragment A on Dog { ...B }
fragment B on Dog { ...A }
{ ...A }
```

Running this through `graphql.ValidateDocument` against any schema produces:

```
runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow
```

The stack points at `collectConflictsBetweenFieldsAndFragment` in `rules_overlapping_fields_can_be_merged.go` recursing on itself.

## Impact

Any server using this library can be crashed by a small malformed query. `NoFragmentCyclesRule` correctly reports the cycle as a validation error, but `OverlappingFieldsCanBeMergedRule` runs in parallel via the visitor and crashes the process before that result is returned.

## Fix

PR #741 proposes a fix.