Bug#25815 Data truncated for column TEXT

Problem: "Data truncated" warning was incorrectly generated
when storing a Japanese character encoded in utf8
into a cp932 column.
Reason: Incorrect wrong warning condition
compared the original length of the character in bytes
(which is 3 in utf8) to the converted length of the
character in bytes (which is 2 in cp932).
Fix: use "how many bytes were scanned from input" instead
of "how many bytes were put to the column" in the condition.

mysql-test/r/ctype_cp932.result

@@ -11353,3 +11353,15 @@ a
 a
 a
 drop table t1;
+set names utf8;
+create table t1 (a text) default character set cp932;
+insert into t1 values (_utf8 0xE38182);
+show warnings;
+Level	Code	Message
+select * from t1;
+a
+あ
+select hex(a) from t1;
+hex(a)
+82A0
+drop table t1;

mysql-test/t/ctype_cp932.test

@@ -413,3 +413,14 @@ select * from t1;
 insert into t1 values ('abc');
 select * from t1;
 drop table t1;
+
+#
+# Bug#25815 Data truncated for column TEXT
+#
+set names utf8;
+create table t1 (a text) default character set cp932;
+insert into t1 values (_utf8 0xE38182);
+show warnings;
+select * from t1;
+select hex(a) from t1;
+drop table t1;

sql/field.cc

@@ -7024,7 +7024,7 @@ int Field_blob::store(const char *from,uint length,CHARSET_INFO *cs)
                               cannot_convert_error_pos, from + length))
     return 2;
 
-  if (copy_length < length)
+  if (from_end_pos < from + length)
   {
     report_data_too_long(this);
     return 2;