Fix addMACD() when user specifies 'col' argument

Calling addMACD(col = ...) would throw the error below.

  R$ chartSeries(x)
  R$ addMACD(col = c("#FF0000", "#008000", "#BBBBBB","#FF0000"))
  Error in ans[ypos] <- rep(yes, length.out = len)[ypos] :
    replacement has length zero
  In addition: Warning message:
  In rep(yes, length.out = len) : 'x' is NULL so the result will be NULL

'col' was being set to NULL if it wasn't missing because there was no
'else' in the ternary-style if statement.

Thanks to @nvalueanalytics for the report.

Fixes #321.

NEWS.md

@@ -1,3 +1,9 @@
+### Changes in 0.4-18.1 (2021-mm-dd)
+
+1. Fix `addMACD()` when `col` is specified. Thanks to @nvalueanalytics for the
+  report!
+  [#321](https://github.com/joshuaulrich/quantmod/issues/321)
+
 ### Changes in 0.4-18 (2020-11-29)
 
 1. Fix issues handling https:// in `getSymbols.yahooj()`. Thanks to @lobo1981

R/addTA.R

@@ -947,8 +947,9 @@ function(x) {
   chobTA <- new("chobTA")
   chobTA@new <- TRUE
 
-  col <- if(missing(col)) col <- c('#999999','#777777',
-                              '#BBBBBB','#FF0000')
+  if(missing(col)) {
+    col <- c('#999999','#777777', '#BBBBBB','#FF0000')
+  }
 
   xx <- if(is.OHLC(x)) {
     Cl(x)