Add reallocating environ if it's set NULL / Fix NULL pointer dereference when PutEnvImpl is called after clearenv

[G4Vi]

Potential fix for #541

Not sure if something different should be done instead of letting the __cxa_atexit(FreeEnviron callbacks stack up.

[jart]

Thank you for sending this!

I'd encourage you to write tests too when encountering issues like this. I'm a big fan of test driven development so the testing tools are pretty high quality to reflect that.

For example, with something like this you'd want to put the following code in test/libc/mem/putenv_test.c

#include "libc/mem/mem.h"
#include "libc/runtime/runtime.h"
#include "libc/testlib/testlib.h"

TEST(putenv, test) {
  EXPECT_EQ(0, clearenv());
  EXPECT_EQ(0, putenv("hi=there"));
  EXPECT_STREQ("there", getenv("hi"));
}

Then you'd run make MODE=dbg -j16 o/dbg/test/libc/mem