[awk] srand return issue?

[jgarzik]

The following failure was provided by another awk author.

Working (macos awk):

% awk 'BEGIN {printf "%s ",srand(1099511627776);print rand();print srand()}'
1 0.268992
1099511627776

Failure (our awk):

% RUST_BACKTRACE=1 ./target/release/awk 'BEGIN {printf "%s ",srand(1099511627776);print rand();print srand()}'
thread 'main' panicked at awk/src/interpreter/mod.rs:1067:36:
empty stack
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::option::expect_failed
   3: awk::interpreter::Stack::pop_scalar_value
   4: awk::interpreter::builtin_sprintf
   5: awk::interpreter::call_simple_builtin
   6: awk::interpreter::Interpreter::run_internal
   7: awk::interpreter::Interpreter::run
   8: awk::interpreter::interpret
   9: awk::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.