Add brackets in SFileSetFilePointer when pointer < 0

This addresses a bug introduced in:

- fe652fe4a6

Brackets were not properly included, causing `SFILE_INVALID_POS` to be
returned whenever `DeltaPos` was less than zero, instead of only when
the resulting position was negative.

src/SFileReadFile.cpp

@@ -868,8 +868,10 @@ DWORD WINAPI SFileSetFilePointer(HANDLE hFile, LONG lFilePos, LONG * plFilePosHi
     if((LONGLONG)DeltaPos < 0)
     {
         if(NewPosition > FileSize) // Position is negative
+        {
             SetLastError(ERROR_NEGATIVE_SEEK);
             return SFILE_INVALID_POS;
+        }
     }
 
     // If moving forward, don't allow the new position go past the end of the file