Add a test case

Distilled from the `melt()` call here:

https://github.com/gdemin/expss/blob/d498caf72a9938ac71b9e06fd61485cd0347d607/tests/testthat/test_subtotal.R#L161-L166

Author: MichaelChirico

inst/tests/tests.Rraw

@@ -21129,3 +21129,8 @@ test(2311.2, nlevels(DT$V1), 2L) # used to be 3
 # avoid translateChar*() in OpenMP threads, #6883
 DF = list(rep(iconv("\uf8", from = "UTF-8", to = "latin1"), 2e6))
 test(2312, fwrite(DF, nullfile(), encoding = "UTF-8", nThread = 2L), NULL)
+
+# avoid memcpy of 0-length inputs
+test(2313,
+     melt(data.table(a=numeric(), b=numeric(), c=numeric()), id.vars=c('a', 'b')),
+     data.table(a=numeric(), b=numeric(), variable=factor(levels='c'), value=numeric()))