MDEV-4241 fix.

Field_enum incorrectly inherited decimals() from Field_string.
Field_enum should be always integer in numeric context.

mysql-test/r/type_enum.result

@@ -1854,3 +1854,18 @@ a
 
 DROP TABLE t1;
 End of 5.1 tests
+#
+# MDEV-4241: Assertion failure: scale >= 0 && precision > 0 &&
+# scale <= precision in decimal_bin_size
+#
+CREATE TABLE t1 (
+f1 enum('1','2','3','4','5')
+) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(2);
+SELECT AVG(f1) FROM t1;
+Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
+def					AVG(f1)	246	7	6	Y	128	4	63
+AVG(f1)
+1.5000
+drop table t1;
+End of 5.3 tests

mysql-test/t/type_enum.test

@@ -221,3 +221,18 @@ SELECT a FROM t1 WHERE a=0;
 DROP TABLE t1;
 
 --echo End of 5.1 tests
+
+--echo #
+--echo # MDEV-4241: Assertion failure: scale >= 0 && precision > 0 &&
+--echo # scale <= precision in decimal_bin_size
+--echo #
+CREATE TABLE t1 (
+  f1 enum('1','2','3','4','5')
+) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(2);
+--enable_metadata
+SELECT AVG(f1) FROM t1;
+--disable_metadata
+drop table t1;
+
+--echo End of 5.3 tests

sql/field.h

@@ -1982,6 +1982,7 @@ class Field_enum :public Field_str {
   bool has_charset(void) const { return TRUE; }
   /* enum and set are sorted as integers */
   CHARSET_INFO *sort_charset(void) const { return &my_charset_bin; }
+  uint decimals() const { return 0; }
 private:
   int do_save_field_metadata(uchar *first_byte);
   uint is_equal(Create_field *new_field);

strings/decimal.c

@@ -1465,7 +1465,9 @@ int decimal_bin_size(int precision, int scale)
       intg0=intg/DIG_PER_DEC1, frac0=scale/DIG_PER_DEC1,
       intg0x=intg-intg0*DIG_PER_DEC1, frac0x=scale-frac0*DIG_PER_DEC1;
 
-  DBUG_ASSERT(scale >= 0 && precision > 0 && scale <= precision);
+  DBUG_ASSERT(scale >= 0);
+  DBUG_ASSERT(precision > 0);
+  DBUG_ASSERT(scale <= precision);
   return intg0*sizeof(dec1)+dig2bytes[intg0x]+
          frac0*sizeof(dec1)+dig2bytes[frac0x];
 }