readDoc function requires of an initialized parameter

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/18)) by Jose Luis Rivero (Bitbucket: [Jose Luis Rivero](https://bitbucket.org/%7Bd12309b2-f745-42ee-b119-aec4fcdf81fe%7D/), GitHub: [j-rivero](https://github.com/j-rivero)).**

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When using the ```sdf::readFile``` or ```sdf::readDoc``` there is a ```sdf::SDFPtr``` parameter which should point to a valid SDF object and has been initialized using ```sdf::init()``` function. Otherwise user will get segfaults or errors.

We should found a better way of handling this need.

Start by tracing sdf::readFile and sdf::readDoc, focusing on their sdf::SDFPtr parameter and the requirement to call sdf::init() first. Determine how an uninitialized parameter currently produces errors or segfaults, then verify that the revised handling avoids that failure without requiring prior initialization.

Written by the indexing model from the issue text.