modcc : report linear test error. (#1278)

Fix segfault in modcc when min/max used in net_receive.

* The error occurred when a synapse that use `max` or `min` expressions in the `net_receive` block: we test for linearity to check whether they can be coalesced; we cannot differentiate the expression; an error is raised but not reported; seg fault. 
* Fixes ` as_c_double` printing which does not print integers as doubles as intended. 

Fixes #1275