add_assignment: don't crash if assignment isn't found

add_assignment previously didn't handle the case where an existing
config assignment was not found, and segfaulted. Fix that by inserting
brand-new assignments at the end of the config list.

src/bin/initdb/initdb.c

@@ -385,6 +385,12 @@ add_assignment(char **lines, const char *varname, const char *fmt, ...)
             iinsert = isrc;
     }
 
+    if (iinsert < 0)
+    {
+        /* No assignment found? Insert at the end. */
+        iinsert = isrc;
+    }
+
     /* Build assignment. */
     va_start(args, fmt);
     if (superseded)