C API: Need to try-catch around new

Per https://en.cppreference.com/w/cpp/memory/new/operator_new.html, it
can throw if the allocation fails.

src/libstore-c/nix_api_store.cc

@@ -215,7 +215,11 @@ void nix_derivation_free(nix_derivation * drv)
 
 StorePath * nix_store_path_clone(const StorePath * p)
 {
+    try {
         return new StorePath{p->path};
+    } catch (...) {
+        return nullptr;
+    }
 }
 
 nix_derivation * nix_derivation_from_json(nix_c_context * context, Store * store, const char * json)

src/libutil-c/nix_api_util.cc

@@ -13,7 +13,11 @@ extern "C" {
 
 nix_c_context * nix_c_context_create()
 {
+    try {
         return new nix_c_context();
+    } catch (...) {
+        return nullptr;
+    }
 }
 
 void nix_c_context_free(nix_c_context * context)