[SPARK-10351][SQL] Fixes UTF8String.fromAddress to handle off-heap memory

[feynmanliang]

CC @rxin @marmbrus

[rxin]

Actually then it looks like the proper fix should make fromAddress handle base = null. Are there existing code that would break from that?

[SparkQA]

Test build #41791 has finished for PR 8523 at commit a967eb8.

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

[feynmanliang]

AFAIK only callers of UTF8String.fromAddress are UnsafeRow.getUTF8String and UnsafeArrayData.getUTF8String, both of which do null checking prior to calling so we can just remove the null checking within fromAddress to fix this

[SparkQA]

Test build #41814 has finished for PR 8523 at commit 198daa7.

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

[rxin]

Thanks - I've merged this.