IMPALA-3018: Fix AllocBuffer() and CopyStringVal() to handle empty strings.

AllocBuffer() and CopyStringVal() are two helper functions used by
various UDAs to allocate buffers for StringVal during their Init()
and Update() functions. Previously, these functions assumed that
the buffer length is always greater than 0. That turned out to be
an invalid assumption. This change removes this assumption and
handles zero-length StringVal by initializing its 'ptr' to NULL and
'len' to 0. A new test is also added to exercise this case.

Change-Id: Ia1e4140376c65ca3c734c40ecc3cce15b8bf2d3f
Reviewed-on: http://gerrit.cloudera.org:8080/2211
Reviewed-by: Dan Hecht <dhecht@cloudera.com>
Tested-by: Internal Jenkins

testdata/workloads/functional-query/queries/QueryTest/aggregation.test

@@ -154,6 +154,15 @@ from alltypesagg where day is not null
 bigint, string, string, string, string
 ====
 ---- QUERY
+# Test for IMPALA-3018. Verify update() function of min() handles
+# zero-length string correctly.
+select min(str) from (values ('aaa' as str), ('')) as tmp
+---- RESULTS
+''
+---- TYPES
+String
+====
+---- QUERY
 # grouping by different data types, with NULLs
 select tinyint_col, count(*) from alltypesagg where day is not null group by 1 order by 1
 ---- RESULTS