When generating gas particles from DM ICs, check that the IDs do not overflow.

src/space.c

@@ -3245,6 +3245,12 @@ void space_generate_gas(struct space *s, const struct cosmology *cosmo,
     p->id = gp_gas->id_or_neg_offset * 2 + 1;
     gp_dm->id_or_neg_offset *= 2;
 
+    if(gp_dm->id_or_neg_offset <= 0)
+      error("DM particle ID overflowd");
+
+    if(p->id <= 0)
+      error("gas particle ID overflowd");
+
     /* Set the links correctly */
     p->gpart = gp_gas;
     gp_gas->id_or_neg_offset = -i;
@@ -3277,7 +3283,7 @@ void space_generate_gas(struct space *s, const struct cosmology *cosmo,
     p->v[2] = gp_gas->v_full[2];
 
     /* Set the smoothing length to the mean inter-particle separation */
-    p->h = d;
+    p->h = 30. * d;
   }
 
   /* Replace the content of the space */