Fixup gdb.python/py-value.exp for bare-metal aarch64-elf

I noticed that testing aarch64-elf gdb with a physical board
ran into issues with gdb.python/py-value.exp. Further investigation showed
that we were actually trying to dereference a NULL pointer (argv) when trying
to access argv[0].

Being bare-metal, argv is not guaranteed to be valid. So we need to make sure
argv is sane before accessing argv[0].

The following patch fixes up the test program to check for a NULL argv and also
improves the testcase a bit so it doesn't have to work with a hardcoded argc
value.

Regression-tested on x86-64 Ubuntu 16.04.

gdb/testsuite/ChangeLog:

2016-10-12  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.python/py-value.c (main): Check if argv is NULL before using it.
	* gdb.python/py-value.exp (test_value_in_inferior): Don't use hardcoded
	argc values.
	Add 1 to argc so we guarantee distinct initial/modified argc values.

1 files changed, 9 insertions, 1 deletions

diff --git a/gdb/testsuite/gdb.python/py-value.c b/gdb/testsuite/gdb.python/py-value.c
index 586a0fd..8a8ace6 100644
--- a/gdb/testsuite/gdb.python/py-value.c
+++ b/gdb/testsuite/gdb.python/py-value.c
@@ -82,7 +82,7 @@ char **save_argv;
 int
 main (int argc, char *argv[])
 {
-  char *cp = argv[0]; /* Prevent gcc from optimizing argv[] out.  */
+  char *cp;
   struct s s;
   union u u;
   PTR x = &s;
@@ -99,6 +99,14 @@ main (int argc, char *argv[])
   const char *sn = 0;
   struct str *xstr;
 
+  /* Prevent gcc from optimizing argv[] out.  */
+
+  /* We also check for a NULL argv in case we are dealing with a target
+     executing in a freestanding environment, therefore there are no
+     guarantees about argc or argv.  */
+  if (argv != NULL)
+    cp = argv[0];
+
   s.a = 3;
   s.b = 5;
   u.a = 7;