Fix array out of bound access

ASAN reports the following error,

(gdb) PASS: gdb.fortran/vla-ptr-info.exp: continue to breakpoint: pvla-associated
print &pvla^M
=================================================================^M
^[[1m^[[31m==14331==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000000ea569f at pc 0x0000008eb546 bp 0x7ffde0c1dc70 sp 0x7ffde0c1dc60^M
^[[1m^[[0m^[[1m^[[34mREAD of size 1 at 0x000000ea569f thread T0^[[1m^[[0m^M
    #0 0x8eb545 in f_print_type(type*, char const*, ui_file*, int, int, type_print_options const*) ../../binutils-gdb/gdb/f-typeprint.c:89^M
    #1 0xb611e2 in type_print(type*, char const*, ui_file*, int) ../../binutils-gdb/gdb/typeprint.c:365^M
    #2 0x7b3471 in c_value_print(value*, ui_file*, value_print_options const*) ../../binutils-gdb/gdb/c-valprint.c:650^M
    #3 0xb99517 in value_print(value*, ui_file*, value_print_options const*) ../../binutils-gdb/gdb/valprint.c:1233^M
    #4 0xa42be8 in print_formatted ../../binutils-gdb/gdb/printcmd.c:321^M
    #5 0xa46ac9 in print_value(value*, format_data const*) ../../binutils-gdb/gdb/printcmd.c:1233^M
    #6 0xa46d82 in print_command_1 ../../binutils-gdb/gdb/printcmd.c:1261^M
    #7 0xa46e3e in print_command ../../binutils-gdb/gdb/printcmd.c:1267

on this line of code

      demangled_args = varstring[strlen (varstring) - 1] == ')';

because varstring is an empty string and strlen () is 0, so "strlen () - 1"
is definitely out of the bound of "varstring",

(gdb) bt 10
    at /home/yao/SourceCode/gnu/gdb/git/gdb/f-typeprint.c:56
    at /home/yao/SourceCode/gnu/gdb/git/gdb/typeprint.c:365
    at /home/yao/SourceCode/gnu/gdb/git/gdb/c-valprint.c:650
    at /home/yao/SourceCode/gnu/gdb/git/gdb/valprint.c:1236

This patch adds a pre-check that varstring is empty or not.

gdb:

2017-02-27  Yao Qi  <yao.qi@linaro.org>

	* f-typeprint.c (f_print_type): Check "varstring" is empty first.

Author: Yao Qi

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2017-02-27  Yao Qi  <yao.qi@linaro.org>
+
+	* f-typeprint.c (f_print_type): Check "varstring" is empty first.
+
 2017-02-26  Alan Hayward  <alan.hayward@arm.com>
 
 	* regcache.c (regcache_raw_update): New function.

gdb/f-typeprint.c

@@ -52,7 +52,6 @@ f_print_type (struct type *type, const char *varstring, struct ui_file *stream,
 	      int show, int level, const struct type_print_options *flags)
 {
   enum type_code code;
-  int demangled_args;
 
   if (type_not_associated (type))
     {
@@ -81,12 +80,15 @@ f_print_type (struct type *type, const char *varstring, struct ui_file *stream,
 
   if (varstring != NULL)
     {
+      int demangled_args;
+
       fputs_filtered (varstring, stream);
 
       /* For demangled function names, we have the arglist as part of the name,
          so don't print an additional pair of ()'s.  */
 
-      demangled_args = varstring[strlen (varstring) - 1] == ')'; 
+      demangled_args = (*varstring != '\0'
+			&& varstring[strlen (varstring) - 1] == ')');
       f_type_print_varspec_suffix (type, stream, show, 0, demangled_args, 0);
    }
 }