[BUGFIX] GeneralUtility::validEmail() can throw warnings

If GeneralUtility::validEmail() is called with array or object
as parameter, warnings or errors are thrown of type "strlen()
expects parameter 1 to be string" or similar.
The patch sanitizes the method by returning early with FALSE if
input parameter is not a string.

Change-Id: I56c8ec91c730e47d341de7f09c14c7ee0492a616
Fixes: #42792
Releases: 6.0
Reviewed-on: http://review.typo3.org/16462
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn