dtc: pos parameter to srcpos_string() can't be NULL

None of the callers ever pass a NULL to srcpos_string(), so the check
for it is not necessary.  Furthermore, checking it make Coverity complain
about the raw dereferences which follow later in the function.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

srcpos.c

@@ -252,7 +252,7 @@ srcpos_string(struct srcpos *pos)
 	const char *fname = "<no-file>";
 	char *pos_str;
 
-	if (pos)
+	if (pos->file && pos->file->name)
 		fname = pos->file->name;