minios: clean up unneeded "err = NULL" in frontend drivers

This patch removes cases where the error message pointer is already
NULL and is then set to NULL. These are harmless, but suggest
incorrect practice: the pointer should be passed to free() to
deallocate memory prior to reassignment. There are no functional
changes in this patch.

Signed-off-by: Ben Cressey <bcressey@amazon.com>
Reviewed-by: Matt Wilson <msw@amazon.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
[msw: split a larger patch from Ben into this cleanup patch]
Signed-off-by: Matt Wilson <msw@amazon.com>

extras/mini-os/blkfront.c

@@ -289,7 +289,6 @@ void shutdown_blkfront(struct blkfront_dev *dev)
                 XenbusStateInitialising, err);
         goto close;
     }
-    err = NULL;
     state = xenbus_read_integer(path);
     while (err == NULL && (state < XenbusStateInitWait || state >= XenbusStateClosed))
         err = xenbus_wait_for_state_change(path, &state, &dev->events);

extras/mini-os/fbfront.c

@@ -272,7 +272,6 @@ void shutdown_kbdfront(struct kbdfront_dev *dev)
                 XenbusStateInitialising, err);
         goto close_kbdfront;
     }
-    err = NULL;
     state = xenbus_read_integer(path);
     while (err == NULL && (state < XenbusStateInitWait || state >= XenbusStateClosed))
     err = xenbus_wait_for_state_change(path, &state, &dev->events);
@@ -664,8 +663,6 @@ void shutdown_fbfront(struct fbfront_dev *dev)
                 XenbusStateInitialising, err);
         goto close_fbfront;
     }
-
-    err = NULL;
     state = xenbus_read_integer(path);
     while (err == NULL && (state < XenbusStateInitWait || state >= XenbusStateClosed))
         err = xenbus_wait_for_state_change(path, &state, &dev->events);

extras/mini-os/netfront.c

@@ -541,7 +541,6 @@ void shutdown_netfront(struct netfront_dev *dev)
                 XenbusStateInitialising, err);
         goto close;
     }
-    err = NULL;
     state = xenbus_read_integer(path);
     while (err == NULL && (state < XenbusStateInitWait || state >= XenbusStateClosed))
         err = xenbus_wait_for_state_change(path, &state, &dev->events);

extras/mini-os/pcifront.c

@@ -355,7 +355,6 @@ void shutdown_pcifront(struct pcifront_dev *dev)
                 XenbusStateInitialising, err);
         goto close_pcifront;
     }
-    err = NULL;
     state = xenbus_read_integer(path);
     while (err == NULL && (state < XenbusStateInitWait || state >= XenbusStateClosed))
         err = xenbus_wait_for_state_change(path, &state, &dev->events);