Fix MI dprintf-insert not printing on a resolved pending location.

Fix MI dprintf-insert not printing on a resolved pending location.

This patch fixes the "Format string required" error when trying to print
a dprintf on a now resolved, pending location when set via the MI interface
even if the format string is entered correctly.

This patch also adds a test case to check that issue called
mi-dprintf-pending.exp.

gdb/ChangeLog:

	PR breakpoints/16465
	* breakpoint.c (create_breakpoint): Save extra_string for
	pending breakpoints.

gdb/testsuite/ChangeLog:

	PR breakpoints/16465
	* gdb.mi/mi-dprintf-pending.c: New file.
	* gdb.mi/mi-dprintf-pending.exp: New test.
	* gdb.mi/mi-dprintf-pendshr.c: New file.