[SPARK-18745][SQL] Fix signed integer overflow due to toInt cast

## What changes were proposed in this pull request?

This PR avoids that a result of a cast `toInt` is negative due to signed integer overflow (e.g. 0x0000_0000_1???????L.toInt < 0 ). This PR performs casts after we can ensure the value is within range of signed integer (the result of `max(array.length, ???)` is always integer).

## How was this patch tested?

Manually executed query68 of TPC-DS with 100TB

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #16235 from kiszk/SPARK-18745.

sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashedRelation.scala

@@ -670,9 +670,9 @@ private[execution] final class LongToUnsafeRowMap(val mm: TaskMemoryManager, cap
     var offset: Long = Platform.LONG_ARRAY_OFFSET
     val end = len * 8L + Platform.LONG_ARRAY_OFFSET
     while (offset < end) {
-      val size = Math.min(buffer.length, (end - offset).toInt)
+      val size = Math.min(buffer.length, end - offset)
       Platform.copyMemory(arr, offset, buffer, Platform.BYTE_ARRAY_OFFSET, size)
-      writeBuffer(buffer, 0, size)
+      writeBuffer(buffer, 0, size.toInt)
       offset += size
     }
   }
@@ -710,8 +710,8 @@ private[execution] final class LongToUnsafeRowMap(val mm: TaskMemoryManager, cap
     var offset: Long = Platform.LONG_ARRAY_OFFSET
     val end = length * 8L + Platform.LONG_ARRAY_OFFSET
     while (offset < end) {
-      val size = Math.min(buffer.length, (end - offset).toInt)
-      readBuffer(buffer, 0, size)
+      val size = Math.min(buffer.length, end - offset)
+      readBuffer(buffer, 0, size.toInt)
       Platform.copyMemory(buffer, Platform.BYTE_ARRAY_OFFSET, array, offset, size)
       offset += size
     }