[BUGFIX] exec_SELECTcountRows returns string

PHPdoc states that return value is either FALSE if
something strange happened or integer with the count of
records.

MySql returns strings, so an intval is needed to have integers

Change-Id: I521db73e7e9a85c2f616d869f50b3d012cdb1dcb
Resolves: #29169
Releases: 4.6, 4.5, 4.4
Reviewed-on: http://review.typo3.org/4540
Reviewed-by: Oliver Klee
Reviewed-by: Xavier Perseguers
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog