MSI: fix some NULL checking in MSI_RecordGetStringA

[Aric Stewart]

Ok,  Though it mirrors what is happening over in MSI_RecordGetStringW so 
i assumed it was just a few check that had been accidentally forgotten.

-aric

James Hawkins wrote:
> On Tue, Dec 16, 2008 at 9:11 AM, Aric Stewart <aric at codeweavers.com> wrote:
>> ---
>>  dlls/msi/record.c |    7 ++++---
>>  1 files changed, 4 insertions(+), 3 deletions(-)
>>
> 
> Can you add tests for this please?
> 
> Thanks,
> James Hawkins
>