Several MTD FLASH drivers nullify the freed 'priv' structure and failed to return NULL as stated in the comments. Result, will operate on a NULL pointer later. Noted by David Sidrane.

drivers/mtd/at25.c

@@ -673,7 +673,7 @@ FAR struct mtd_dev_s *at25_initialize(FAR struct spi_dev_s *dev)
 
           ferr("ERROR: Unrecognized\n");
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
       else
         {

drivers/mtd/is25xp.c

@@ -978,7 +978,7 @@ FAR struct mtd_dev_s *is25xp_initialize(FAR struct spi_dev_s *dev)
 
           ferr("ERROR: Unrecognized\n");
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
       else
         {
@@ -999,4 +999,3 @@ FAR struct mtd_dev_s *is25xp_initialize(FAR struct spi_dev_s *dev)
   finfo("Return %p\n", priv);
   return (FAR struct mtd_dev_s *)priv;
 }
-

drivers/mtd/m25px.c

@@ -1033,7 +1033,7 @@ FAR struct mtd_dev_s *m25p_initialize(FAR struct spi_dev_s *dev)
 
           ferr("ERROR: Unrecognized\n");
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
     }
 

drivers/mtd/ramtron.c

@@ -826,7 +826,7 @@ FAR struct mtd_dev_s *ramtron_initialize(FAR struct spi_dev_s *dev)
           /* Unrecognized! Discard all of that work we just did and return NULL */
 
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
     }
 

drivers/mtd/sst25.c

@@ -1208,7 +1208,7 @@ FAR struct mtd_dev_s *sst25_initialize(FAR struct spi_dev_s *dev)
 
           ferr("ERROR: Unrecognized\n");
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
       else
         {

drivers/mtd/sst25xx.c

@@ -956,7 +956,7 @@ FAR struct mtd_dev_s *sst25xx_initialize(FAR struct spi_dev_s *dev)
 
           ferr("ERROR: Unrecognized\n");
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
       else
         {

drivers/mtd/sst26.c

@@ -934,7 +934,7 @@ FAR struct mtd_dev_s *sst26_initialize_spi(FAR struct spi_dev_s *dev)
 
           ssterr("ERROR: Unrecognized\n");
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
       else
         {

drivers/mtd/w25.c

@@ -1258,7 +1258,7 @@ FAR struct mtd_dev_s *w25_initialize(FAR struct spi_dev_s *spi)
 
           ferr("ERROR: Unrecognized\n");
           kmm_free(priv);
-          priv = NULL;
+          return NULL;
         }
       else
         {