Fix latent quote char bug in generic_printstr

generic_printstr prints an empty string like:

      fputs_filtered ("\"\"", stream);

However, this seems wrong to me if the quote character is something
other than double quote.  This patch fixes this latent bug.  Thanks to
Andrew for the test case.

Co-authored-by: Andrew Burgess <aburgess@redhat.com>

gdb/testsuite/gdb.fortran/empty-string.exp

@@ -0,0 +1,33 @@
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/> .
+
+# Test printing of an empty Fortran string.
+
+if {[skip_fortran_tests]} { return -1 }
+
+standard_testfile ".f90"
+load_lib fortran.exp
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug f90}]} {
+    return -1
+}
+
+if ![fortran_runto_main] {
+    return -1
+}
+
+gdb_breakpoint [gdb_get_line_number "Break here"]
+gdb_continue_to_breakpoint "run to test location"
+gdb_test "print string" " = ''"

gdb/testsuite/gdb.fortran/empty-string.f90

@@ -0,0 +1,30 @@
+! Copyright 2022 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+program empty_string
+  implicit none
+
+  integer :: ret
+
+  ret = string_length('')
+
+contains
+
+  integer(kind=4) function string_length(string)
+    character*(*) :: string
+    string_length = len(string)		! Break here.
+  end function string_length
+
+end program empty_string

gdb/valprint.c

@@ -2516,7 +2516,7 @@ generic_printstr (struct ui_file *stream, struct type *type,
 
   if (length == 0)
     {
-      gdb_puts ("\"\"", stream);
+      gdb_printf (stream, "%c%c", quote_char, quote_char);
       return;
     }