Editline: Fix an msan error

[labath]

Summary

Despite the documentation for the el_get(EL_GETTC) function claiming the
vararg part is (const char *name, void *value), in reality the function
expects the vararg list to be terminated by a null pointer, which can be
clearly seen by examining the source code. Although this is mostly
bening because the extra values are not used it any way, it still lights
up as an error when running the tests under msan.

Work around this quirk by adding an explicit nullptr to the end of the
argument list.

[labath]

You can see the relevant part of the source code here https://github.com/cdesjardins/libedit/blob/master/src/eln.c#L310 (not the canonical source, but at least it's easy to link to).

@christos, any chance of getting this fixed (either the docs, or the source code)?

[davide]

LGTM but this should really be fixed upstream (in libedit)

source/Host/common/Editline.cpp

void Editline::SetContinuationPrompt(const char *continuation_prompt) {
   m_set_continuation_prompt =
       continuation_prompt == nullptr ? "" : continuation_prompt;
 }
 
 void Editline::TerminalSizeChanged() {
   if (m_editline != nullptr) {
     el_resize(m_editline);
     int columns;
-    // Despite the man page claiming non-zero indicates success, it's actually
-    // zero
-    if (el_get(m_editline, EL_GETTC, "co", &columns) == 0) {
+    // This function is documenting as taking (const char *, void *) for the
+    // vararg part, but in reality in was consuming arguments until the first
+    // null pointer. This was fixed in libedit in April 2019
+    // <http://mail-index.netbsd.org/source-changes/2019/04/26/msg105454.html>,
+    // but we're keeping the workaround until a version with that fix is more
+    // widely available.
+    if (el_get(m_editline, EL_GETTC, "co", &columns, nullptr) == 0) {
       m_terminal_width = columns;
       if (m_current_line_rows != -1) {
         const LineInfoW *info = el_wline(m_editline);
         int lineLength =
             (int)((info->lastchar - info->buffer) + GetPromptWidth());
         m_current_line_rows = (lineLength / columns) + 1;
       }
     } else {