tool_parsecfg: make get_line handle lines ending on the buffer boundary

Add test 743 to verify.

Fixes #17030
Reported-by: Marius Kleidl
Closes #17031

src/tool_parsecfg.c

@@ -312,6 +312,8 @@ static bool get_line(FILE *input, struct dynbuf *buf, bool *error)
       else if(feof(input))
         return TRUE; /* all good */
     }
+    else if(curlx_dyn_len(buf))
+      return TRUE; /* all good */
     else
       break;
   }