R_OpenSiteFile: Do not use a null ptr as filename in fopen (PR#312)

[hoffmann@ehmgs2.et.tu-dresden.de]

I get crashes when I call R without any arguments and having no
environment variables R_PROFILE and RPROFILE.

The reason is, that in unix/sys-common.c in R_OpenSiteFile() fopen 
is called with a null ptr as filename (because this is the outcome 
of getenv()).

Although most compilers seem to interpret this as "file not found",
this is an undefined behavior concerning to all C standards I could get
hold of.

A request in comp.lang.c gave the following response:
...
Your program should crash. If you might get a NULL pointer as
a filename, you should have a conditional statement to handle
that case.
...

So I would suggest to test for the return value of getenv, before 
using it as filename in fopen.

--please do not edit the information below--

Version:
 platform = hppa1.1-hp-hpux10.20
 arch = hppa1.1
 os = hpux10.20
 system = hppa1.1, hpux10.20
 status = 
 status.rev = 0
 major = 0
 minor = 65.0
 year = 1999
 month = August
 day = 27
 language = R

Search Path:
 .GlobalEnv, Autoloads, package:base

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