Bug 1661378 - pkix: Do not use NULL where 0 is needed

Clang finds this error

pkix_logger.c:316:32: error: cast to smaller integer type 'PKIX_ERRORCLASS' from 'void *' [-Werror,-Wvoid-pointer-to-enum-cast]
        logger->logComponent = (PKIX_ERRORCLASS)NULL;
                               ^~~~~~~~~~~~~~~~~~~~~
pkix_logger.c:617:32: error: cast to smaller integer type 'PKIX_ERRORCLASS' from 'void *' [-Werror,-Wvoid-pointer-to-enum-cast]
        logger->logComponent = (PKIX_ERRORCLASS)NULL;
                               ^~~~~~~~~~~~~~~~~~~~~
2 errors generated.

Signed-off-by: Khem Raj <raj.khem@gmail.com>

lib/libpkix/pkix/util/pkix_logger.c

@@ -313,7 +313,7 @@ pkix_Logger_Destroy(
 
         logger->callback = NULL;
         PKIX_DECREF(logger->context);
-        logger->logComponent = (PKIX_ERRORCLASS)NULL;
+        logger->logComponent = (PKIX_ERRORCLASS)0;
 
 cleanup:
 
@@ -614,7 +614,7 @@ PKIX_Logger_Create(
 
         logger->callback = callback;
         logger->maxLevel = 0;
-        logger->logComponent = (PKIX_ERRORCLASS)NULL;
+        logger->logComponent = (PKIX_ERRORCLASS)0;
 
         PKIX_INCREF(loggerContext);
         logger->context = loggerContext;