[fixed] check before react-modal removal from parent element that par…

[dremlin2000]

Fixes #820.

Changes proposed:
Check if this.node element exists in parent element before removal

Upgrade Path (for changed or removed APIs):
N/A

Acceptance Checklist:

• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage remained the same at 87.135% when pulling 1094ce1 on dremlin2000:bug/patch-1 into bd07d56 on reactjs:master.

[diasbruno]

Thanks, @dremlin2000!