CTLNUL leakage in bash-20190220

[Grisha Levit]

Since the changes in [1], there's an issue with CTLNUL leakage in
${param=word} when word has null strings, like:

    $ unset v; printf '<%q>' ${v= ''}
    <$'\177'>

I think this can be addressed with:

diff --git a/subst.c b/subst.c
index 4962d2f5..58d33996 100644
--- a/subst.c
+++ b/subst.c
@@ -10314,7 +10314,7 @@ add_twochars:
             this is when we are going to be performing word splitting,
             since we have to preserve a null argument if the next character
             will cause word splitting. */
-         if (temp == 0 && quoted_state == PARTIALLY_QUOTED && quoted
== 0 && (word->flags & W_NOSPLIT) == 0 && (word->flags & W_EXPANDRHS))
+         if (temp == 0 && quoted_state == PARTIALLY_QUOTED && quoted
== 0 && (word->flags & W_NOSPLIT) == 0 && (word->flags & W_EXPANDRHS)
&& (word->flags & W_ASSIGNRHS) == 0)
            {
              c = CTLNUL;
              sindex--;
@@ -10373,7 +10373,7 @@ add_twochars:
             partially quoted; such nulls are discarded.  See above for the
             exception, which is when the string is going to be split.
             Posix interp 888/1129 */
-         if (temp == 0 && quoted_state == PARTIALLY_QUOTED && quoted
== 0 && (word->flags & W_NOSPLIT) == 0 && (word->flags & W_EXPANDRHS))
+         if (temp == 0 && quoted_state == PARTIALLY_QUOTED && quoted
== 0 && (word->flags & W_NOSPLIT) == 0 && (word->flags & W_EXPANDRHS)
&& (word->flags & W_ASSIGNRHS) == 0)
            {
              c = CTLNUL;
              sindex--;


    [1]: 
https://git.savannah.gnu.org/cgit/bash.git/commit/?h=devel&id=9e48f856544da2d2cc95600f487e5b5bcefa0d85