[SPARK-11827][SQL] Fix compilation against Java 7

[ted-yu]

What changes were proposed in this pull request?

This PR fixes compilation error against Java 7:

sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala:137: value longValueExact is not a member of java.math.BigInteger
[ERROR] this.longVal = bigintval.longValueExact()
[ERROR] ^
[ERROR] one error found

How was this patch tested?

Existing tests.

[rxin]

@srowen are you saying this fix is not sufficient?

[srowen]

Yes, this fails to check that the value fits in a long and will silently truncate.

[ted-yu]

Should reflection be used :-)

[srowen]

No, it's as simple as the fix in #13223 This PR should be closed.

[SparkQA]

Test build #59028 has finished for PR 13233 at commit 422ccad.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.