loci: workaround a clang analyzer warning

Clang figured out that in the case of a fixed-length object the `len =
wire_len` assignment was redundant.
diff --git a/c_gen/templates/loci_validator.c b/c_gen/templates/loci_validator.c
index a275029..1f98b21 100644
--- a/c_gen/templates/loci_validator.c
+++ b/c_gen/templates/loci_validator.c
@@ -110,8 +110,11 @@
         return -1;
     }
 
+:: if not ofclass.is_fixed_length:
     len = wire_len;
 
+:: #endif
+::
 :: elif type(m) == OFFieldLengthMember:
 :: # Read field length members
 :: field_length_members[m.field_name] = m