awk: fix segfault on closing non-opened file

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -77,6 +77,12 @@ testing "awk string cast (bug 725)" \
 testing "awk handles whitespace before array subscript" \
 	"awk 'BEGIN { arr [3] = 1; print arr [3] }'" "1\n" "" ""
 
+# GNU awk 3.1.5's "print ERRNO" prints "No such file or directory" instead of "2",
+# do we need to emulate that as well?
+testing "awk handles non-existing file correctly" \
+	"awk 'BEGIN { getline line <\"doesnt_exist\"; print ERRNO; ERRNO=0; close(\"doesnt_exist\"); print ERRNO; print \"Ok\" }'" \
+	"2\n0\nOk\n" "" ""
+
 prg='
 BEGIN {
   u["a"]=1