Bug #27164: not reseting the data pointer

 to 0 causes wrong (large) length to be read
 from the row in _mi_calc_blob_length() when 
 storing NULL values in (e.g) POINT columns.
 This large length is then used to allocate
 a block of memory that (on some OSes) causes
 trouble.
 Fixed by calling the base class's 
 Field_blob::reset() from Field_geom::reset()
 that is called when storing a NULL value into
 the column.

mysql-test/r/gis.result

@@ -718,4 +718,10 @@ point(b, b) IS NULL	linestring(b) IS NULL	polygon(b) IS NULL	multipoint(b) IS NU
 1	1	1	1	1	1	1
 0	1	1	1	1	1	1
 drop table t1;
+CREATE TABLE t1(a POINT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (NULL);
+SELECT * FROM t1;
+a
+NULL
+DROP TABLE t1;
 End of 4.1 tests

mysql-test/t/gis.test

@@ -411,4 +411,12 @@ from t1;
 
 drop table t1;
 
+#
+# Bug #27164: Crash when mixing InnoDB and MyISAM Geospatial tables
+#
+CREATE TABLE t1(a POINT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (NULL);
+SELECT * FROM t1;
+DROP TABLE t1;
+
 --echo End of 4.1 tests

sql/field.h

@@ -1115,7 +1115,7 @@ class Field_geom :public Field_blob {
   int  store(const char *to, uint length, CHARSET_INFO *charset);
   int  store(double nr) { return 1; }
   int  store(longlong nr) { return 1; }
-  int  reset(void) { return !maybe_null(); }
+  int  reset(void) { return !maybe_null() || Field_blob::reset(); }
 
   void get_key_image(char *buff,uint length, CHARSET_INFO *cs,imagetype type);
   void set_key_image(char *buff,uint length, CHARSET_INFO *cs);