Fixed BUG#3132: Stored function returns empty when used in union.

mysql-test/r/sp.result:
  Added testcase for BUG#3132.
mysql-test/t/sp.test:
  Added testcase for BUG#3132.
sql/item_func.cc:
  Set a useful max_length in Item_func_sp so field widths are correctly calculated.

mysql-test/r/sp.result

@@ -1353,6 +1353,13 @@ drop procedure bug2564_1|
 drop procedure bug2564_2|
 drop function bug2564_3|
 drop function bug2564_4|
+create function bug3132(s char(20)) returns char(50)
+return concat('Hello, ', s, '!')|
+select bug3132('Bob') union all select bug3132('Judy')|
+bug3132('Bob')
+Hello, Bob!
+Hello, Judy!
+drop function bug3132|
 drop table if exists fac|
 create table fac (n int unsigned not null primary key, f bigint unsigned)|
 create procedure ifac(n int unsigned)

mysql-test/t/sp.test

@@ -1551,6 +1551,15 @@ drop procedure bug2564_2|
 drop function bug2564_3|
 drop function bug2564_4|
 
+#
+# BUG#3132
+#
+create function bug3132(s char(20)) returns char(50)
+  return concat('Hello, ', s, '!')|
+
+select bug3132('Bob') union all select bug3132('Judy')|
+drop function bug3132|
+
 
 #
 # Some "real" examples

sql/item_func.cc

@@ -3320,7 +3320,7 @@ Item_func_sp::fix_length_and_dec()
     switch (m_sp->result()) {
     case STRING_RESULT:
       maybe_null= 1;
-      max_length= 0;
+      max_length= MAX_BLOB_WIDTH;
       break;
     case REAL_RESULT:
       decimals= NOT_FIXED_DEC;