disk/part.c: Fix device enumeration through API

The patch below fixes device enumeration through the U-Boot API.

Device enumeration crashes when the system in question doesn't
have any RAM mapped to address zero (I discovered this on a
BeagleBone board), since the enumeration calls get_dev with a
NULL ifname sometimes which then gets passed down to strncmp().

This fix simply ensures that get_dev returns NULL when invoked
with a NULL ifname.

Signed-off-by: Tim Kientzle <kientzle@freebsd.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>

disk/part.c

@@ -80,6 +80,9 @@ block_dev_desc_t *get_dev(char* ifname, int dev)
 	block_dev_desc_t* (*reloc_get_dev)(int dev);
 	char *name;
 
+	if (!ifname)
+		return NULL;
+
 	name = drvr->name;
 #ifdef CONFIG_NEEDS_MANUAL_RELOC
 	name += gd->reloc_off;