thunk: assert nb_fields is valid

thunk.c:91:32: warning: Call to 'malloc' has an allocation size of 0 bytes
        se->field_offsets[i] = malloc(nb_fields * sizeof(int));
                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

thunk.c

@@ -67,7 +67,6 @@ void thunk_register_struct(int id, const char *name, const argtype *types)
     int nb_fields, offset, max_align, align, size, i, j;
 
     assert(id < max_struct_entries);
-    se = struct_entries + id;
 
     /* first we count the number of fields */
     type_ptr = types;
@@ -76,6 +75,8 @@ void thunk_register_struct(int id, const char *name, const argtype *types)
         type_ptr = thunk_type_next(type_ptr);
         nb_fields++;
     }
+    assert(nb_fields > 0);
+    se = struct_entries + id;
     se->field_types = types;
     se->nb_fields = nb_fields;
     se->name = name;