[PATCH] ecpg: fix missing NULL check in ecpg_store_input

Hi all,

This is my first contribution, so please correct me if anything
is wrong.

During OOM, PGTYPESnumeric_to_asc() can return NULL even on valid
inputs, which can lead to a NULL deref in ecpg_store_input():

```
str = PGTYPESnumeric_to_asc(nval, nval->dscale);
slen = strlen(str);
```

This patch adds the NULL check, matching the surrounding code. This is
unlikely to be hit in practice, so it's more of a hardening than
a real fix.

---
Ilya Gladyshev // foxido.dev