floor(NaN) problem fixed in massdist.c (PR#291)

This will fix the "problem 2 (crash in fft)" in Bug ID #277

On Linux/Alpha, make check failed because R could not handle the following
example in base-Ex.R

##___ Examples ___:

# The Old Faithful geyser data
data(faithful)
 :
 :
## Missing values:
x <- xx <- faithful$eruptions
x[i.out <- sample(length(x), 10)] <- NA
doR <- density(x, bw=0.15, na.rm = TRUE)
doN <- density(x, bw=0.15, na.rm = FALSE)
	### it fails at this statement ###
lines(doR, col="blue")
...

The following statement in density() was not getting the right output:
       
    y <- .C("massdist", x = as.double(x), nx = N, xlo = as.double(lo), 
        xhi = as.double(up), y = double(2 * n), ny = as.integer(n), 
        NAOK = has.na, PACKAGE = "base")$y

y[1] and y[2] should be 0 and 0 but both of them became NaN after this
statement.

So I looked at src/appl/masdist.c with a debugger.  The bug is caused
by floor().  On Alpha/Linux, floor(NaN) returns 0.

 x[i] in the
following code is the list passed from the above statement in
density().  The array x contains a couple of NaN.  When x[i] is NaN,
xpos become NaN.  Then ix = floor(xpos) returns an unexpected result.
On linux/i386, ix become -2147483648 when xpos = NaN(0x80000000007a2).
However ix become 0 on Linux/Alpha when xpos=NaN.  When xpos = NaN, it
shouldn't go into the following "if" statement.  But it goes inside on
Alpha/Linux (because ixmin is 0), and set y[0] and y[1] to NaN.

Following is extracted from massdist.c

    for(i=0 ; i<*nx ; i++) {
    	xpos = (x[i] - *xlow) / xdelta;
    	ix = floor(xpos);
    	fx = xpos - ix;
    	if(ixmin <= ix && ix <= ixmax) {
    	    y[ix] += (1.0 - fx);
    	    y[ix + 1] += fx;
    	}
    	else if(ix == -1) {
    	    y[0] += fx;
    	}
    	else if(ix == ixmax + 1) {
    	    y[ixmax + 1] += (1.0 - fx);
    	}
    }


So I think the statement

	if(ixmin <= ix && ix <= ixmax) {

should be changed to 

    	if((! isnan(ix)) && ixmin <= ix && ix <= ixmax) {

or 
........