[Minor] [SQL] [SPARK-6729] Minor fix for DriverQuirks get

[vlyubin]

The function uses .substring(0, X), which will trigger OutOfBoundsException if string length is less than X. A better way to do this is to use startsWith, which won't error out in this case.

[SparkQA]

Test build #29761 has started for PR 5378 at commit 504e8e0.

[SparkQA]

Test build #29761 has finished for PR 5378 at commit 504e8e0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/29761/
Test PASSed.

[aarondav]

LGTM, merging into master. Thanks!