Escape the error string in process expansion errors

This prevents `echo %*` from printing a private-use character in the
error string.

Fixes #1720.

1 files changed, 1 insertions, 1 deletions

diff --git a/expand.cpp b/expand.cpp
index 3851d6b2..351a84f1 100644
--- a/expand.cpp
+++ b/expand.cpp
@@ -860,7 +860,7 @@ static bool expand_pid(const wcstring &instr_with_sep, expand_flags_t flags, std
         if (!(flags & ACCEPT_INCOMPLETE))
         {
             /* We failed to find anything */
-            append_syntax_error(errors, 1, FAILED_EXPANSION_PROCESS_ERR_MSG, in+1);
+            append_syntax_error(errors, 1, FAILED_EXPANSION_PROCESS_ERR_MSG, escape(in+1, ESCAPE_NO_QUOTED).c_str());
             return false;
         }
     }