gdb.base/fileio.exp: Remove nowarnings

... and quiet -Wnonnull in a different way.

gdb/testsuite/ChangeLog:
2017-05-18  Pedro Alves  <palves@redhat.com>

	* gdb.base/fileio.c (null_str): New global.
	(test_stat): Use it.
	* gdb.base/fileio.exp: Remove nowarnings.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2017-05-18  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/fileio.c (null_str): New global.
+	(test_stat): Use it.
+	* gdb.base/fileio.exp: Remove nowarnings.
+
 2017-05-17  Yao Qi  <yao.qi@linaro.org>
 
 	* gdb.base/fileio.exp: Pass nowarnings to gdb_compile.

gdb/testsuite/gdb.base/fileio.c

@@ -76,6 +76,10 @@ static const char *strerrno (int err);
 
 static void stop () {}
 
+/* A NULL string.  We pass this to stat below instead of a NULL
+   literal to avoid -Wnonnull warnings.  */
+const char *null_str;
+
 int
 test_open ()
 {
@@ -294,7 +298,7 @@ test_stat ()
   stop ();
   /* NULL pathname */
   errno = 0;
-  ret = stat (NULL, &st);
+  ret = stat (null_str, &st);
   printf ("stat 2: ret = %d, errno = %d %s\n", ret, errno,
   	  strerrno (errno));
   stop ();

gdb/testsuite/gdb.base/fileio.exp

@@ -31,7 +31,7 @@ if {[is_remote host]} {
 
 if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" \
 	   executable \
-	   [list debug nowarnings "additional_flags=-DOUTDIR=\"$outdir/\""]] != "" } {
+	   [list debug "additional_flags=-DOUTDIR=\"$outdir/\""]] != "" } {
     untested "failed to compile"
     return -1
 }