Fix recent regression in mcols() setter

The regression was introduced at commit 05a99cd and it breaks the mcols()
setter when the right value is a zero-column ordinary data frame.
This affected several Bioconductor software packages: Gviz, chimeraviz,
rtracklayer, AllelicImbalance, ORFik, and a few others...

DESCRIPTION

@@ -8,7 +8,7 @@ Description: The S4Vectors package defines the Vector and List virtual classes
 	interest (e.g. DataFrame, Rle, and Hits) are implemented in the
 	S4Vectors package itself (many more are implemented in the IRanges
 	package and in other Bioconductor infrastructure packages).
-Version: 0.23.24
+Version: 0.23.25
 Encoding: UTF-8
 Author: H. Pagès, M. Lawrence and P. Aboyoun
 Maintainer: Bioconductor Package Maintainer <maintainer@bioconductor.org>

R/DataFrame-class.R

@@ -180,10 +180,12 @@ setValidity2("DataFrame", .valid.DataFrame)
 ###
 
 ### Low-level constructor. For internal use only.
-### Note that, when supplied, 'nrows' is trusted.
-### Calling 'new_DataFrame(x)' on an ordinary named list 'x' will turn it
-### into a DataFrame in the most possibly straightforward way, that is,
-### calling 'as.list()' on the result will return a list identical to 'x'.
+### Note that, when supplied, 'nrows' is trusted (except when 'listData' is a
+### data.frame or data-frame-like object).
+### Calling 'new_DataFrame(x)' on an ordinary named list or an ordinary
+### data.frame 'x' will turn it into a DataFrame in the most possibly
+### straightforward way. In particular calling 'as.list()' or 'as.data.frame()'
+### on the returned DataFrame will bring back 'x'.
 ### This is unlike 'DataFrame(x)' or 'as(x, "DataFrame")' which can do all
 ### kind of hard-to-predict mangling to 'x', unless the user does something
 ### like 'DataFrame(lapply(x, I))'. Not super convenient or intuitive!
@@ -193,19 +195,32 @@ new_DataFrame <- function(listData=list(), nrows=NA, what="arguments")
         stopifnot(isSingleNumberOrNA(nrows))
         if (!is.integer(nrows))
             nrows <- as.integer(nrows)
-        if (length(listData) == 0L) {
-            if (is.na(nrows))
-                nrows <- 0L
-            names(listData) <- character(0)
+        listData_nrow <- nrow(listData)
+        if (is.null(listData_nrow)) {
+            ## 'listData' is NOT a data.frame or data-frame-like object.
+            if (length(listData) == 0L) {
+                if (is.na(nrows))
+                    nrows <- 0L
+                names(listData) <- character(0)
+            } else {
+                if (is.na(nrows)) {
+                    elt_nrows <- elementNROWS(listData)
+                    nrows <- elt_nrows[[1L]]
+                    if (!all(elt_nrows == nrows))
+                        stop(wmsg(what, " imply differing number of rows"))
+                }
+                if (is.null(names(listData)))
+                    names(listData) <- paste0("V", seq_along(listData))
+            }
         } else {
+            ## 'listData' is a data.frame or data-frame-like object.
             if (is.na(nrows)) {
-                elt_nrows <- elementNROWS(listData)
-                nrows <- elt_nrows[[1L]]
-                if (!all(elt_nrows == nrows))
-                    stop(wmsg(what, " imply differing number of rows"))
+                nrows <- listData_nrow
+            } else if (nrows != listData_nrow) {
+                stop(wmsg("the supplied 'nrows' does not match ",
+                          "the nb of rows in 'listData'"))
             }
-            if (is.null(names(listData)))
-                names(listData) <- paste0("V", seq_along(listData))
+            listData <- as.list(listData)
         }
         new2("DFrame", nrows=nrows, listData=listData, check=FALSE)
 }