Using Clang's static analyzer on bash

[Eduardo Bustamante]

I built bash using scan-build
(https://clang-analyzer.llvm.org/scan-build.html) and I noticed that
it was able to detect the null pointer dereference reported earlier by
Jaren (https://lists.gnu.org/archive/html/bug-bash/2017-04/msg00100.html).

dualbus@debian:~/src/gnu/bash$ scan-build-3.9 make
scan-build: Using '/usr/lib/llvm-3.9/bin/clang' for static analysis

[...]

bashline.c:2548:7: warning: Null pointer argument in call to string
comparison function
  if (strcmp (new_line, rl_line_buffer) != 0)
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/x86_64-linux-gnu/bits/string2.h:125:15: note: expanded
from macro 'strcmp'
            : __builtin_strcmp (s1, s2)))); })
              ^~~~~~~~~~~~~~~~~~~~~~~~~

scan-build currently picks up 263 "bugs" from the devel branch. Maybe
there's value in reducing that number to 0. I think it could be useful
to uncover new bugs.