limits-util: add assert for physical memory calculation overflow

Coverity flags (uint64_t) sc * (uint64_t) ps as a potential
overflow. Add an assert to make the bounds explicit for static
analyzers.

CID#1548042

Follow-up for eefc66aa8f

src/basic/limits-util.c

@@ -28,6 +28,8 @@ uint64_t physical_memory(void) {
         assert(sc > 0);
 
         ps = page_size();
+        /* Silence static analyzers */
+        assert((uint64_t) sc <= UINT64_MAX / (uint64_t) ps);
         mem = (uint64_t) sc * (uint64_t) ps;
 
         r = cg_get_root_path(&root);