Fix compiler warning on Windows.

Commit 41c912ca caused MSVC to complain that not all control paths
return a value.

Thomas Munro

Discussion: https://postgr.es/m/CAEepm%3D3%3DTDYEXUEcHpEx%2BTwc31wo7PA0oBAiNt6sWmq93MW02A%40mail.gmail.com

src/bin/pgbench/pgbench.c

@@ -1980,7 +1980,8 @@ evalStandardFunc(TState *thread, CState *st,
 					}
 				}
 
-				break;			/* NOTREACHED */
+				Assert(0);
+				return false;	/* NOTREACHED */
 			}
 
 			/* integer bitwise operators */