Fix alias command not detecting non matching prefix & sometimes asserting.

Fix alias command not detecting non matching prefix & sometimes asserting.

alias_command does not detect that the prefixes of the alias command and the
aliased command are not matching: it is comparing the alias prefix
with itself, instead of comparing it with the aliased command prefix.

This causes either the alias command to silently do nothing,
or to have GDB asserting:
    (gdb) alias assigne imprime limite-elements = set print elements
    ../../binutils-gdb/gdb/cli/cli-cmds.c:1552: internal-error: void alias_command(const char*, int): Assertion `c_command != NULL && c_command != (struct cmd_list_element *) -1' failed.
    A problem internal to GDB has been detected,

Fix the logic, and update gdb.base/alias.exp to test these cases.

gdb/ChangeLog
2019-06-25  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* cli/cli-cmds.c (alias_command): Compare the alias prefix
	with the command prefix.

gdb/testsuite/ChangeLog
2019-06-25  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.base/alias.exp: Test non matching/non existing prefixes.