Fix spurious FAILs with examine-backward.exp

Fix spurious FAILs with examine-backward.exp

The test case examine-backward.exp issues the command "x/-s" after the end
of the first string in TestStrings, but without making sure that this
string is preceded by a string terminator.  Thus GDB may spuriously print
some random characters from before that string, and then the test fails.

This patch assures that TestStrings is preceded by a string terminator.

gdb/testsuite/ChangeLog:

	* gdb.base/examine-backward.c (Barrier): New character array
	constant, to ensure that TestStrings is preceded by a string
	terminator.