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.

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