Potential buffer under-run in shell_execve()

[John E. Malmberg]

Hello,

In Bash 4.3.42:

In execute_cmd/shell_execve(), if HAVE_BASH_BANG_EXEC is defined, the 
macro READ_SAMPLE_BUF has the potential to set sample_len to -1.

#if defined (HAVE_HASH_BANG_EXEC)
          READ_SAMPLE_BUF (command, sample, sample_len);
          sample[sample_len - 1] = '\0';

This would cause sample[-2] to be set to 0.  Most likely it would set 
part of fd to 0, but all that depends on the compiler.

Since fd is not in use at this point, the under run would not be noticed.

Regards,
-John