Fix coverity issues

Fixes coverity issues 1642964, 1642965, 1642966, 1642968, 1642969

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/26793)

providers/implementations/skeymgmt/generic.c

@@ -45,6 +45,9 @@ void *generic_import(void *provctx, int selection, const OSSL_PARAM params[])
         return NULL;
 
     generic = OPENSSL_zalloc(sizeof(PROV_SKEY));
+    if (generic == NULL)
+        return NULL;
+
     generic->libctx = libctx;
 
     generic->type = SKEY_TYPE_GENERIC;