SEGV when make is run with malformed environment

Hello,

GNU Make crashes with a SEGV when the environment contains invalid entries.
For example:

```c
#include <unistd.h>

int main(void)
{
    char *envp[] = {"key", NULL};    // key without value
    execve("/bin/make", (char *[]) {"/bin/make", NULL}, envp);
}
```

Regards,
OUBARKA