mkimg: Check for realloc failure

If realloc fails, clean up and return ENOMEM; don't just copy data
into NULL.

Reviewed by:	emaste
Reported by:	Claude Opus 5.5
MFC after:	2 weeks
Sponsored by:	Amazon
Differential Revision:	https://reviews.freebsd.org/D60147

usr.bin/mkimg/vmdk.c

@@ -133,7 +133,12 @@ vmdk_write(int fd)
 		return (ENOMEM);
 
 	desc_len = (n + VMDK_SECTOR_SIZE - 1) & ~(VMDK_SECTOR_SIZE - 1);
-	desc = realloc(desc, desc_len);
+	buf = realloc(desc, desc_len);
+	if (buf == NULL) {
+		free(desc);
+		return (ENOMEM);
+	}
+	desc = buf;
 	memset(desc + n, 0, desc_len - n);
 
 	le64enc(&hdr.desc_offset, 1);