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.

1 files changed, 5 insertions, 4 deletions

diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index 62f4d7f..5124ab3 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -1518,8 +1518,8 @@ alias_command (const char *args, int from_tty)
      Example: alias spe = set print elements
 
      Otherwise ALIAS and COMMAND must have the same number of words,
-     and every word except the last must match; and the last word of
-     ALIAS is made an alias of the last word of COMMAND.
+     and every word except the last must identify the same prefix command;
+     and the last word of ALIAS is made an alias of the last word of COMMAND.
      Example: alias set print elms = set pr elem
      Note that unambiguous abbreviations are allowed.  */
 
@@ -1538,10 +1538,11 @@ alias_command (const char *args, int from_tty)
 	error (_("Mismatched command length between ALIAS and COMMAND."));
 
       /* Create copies of ALIAS and COMMAND without the last word,
-	 and use that to verify the leading elements match.  */
+	 and use that to verify the leading elements give the same
+	 prefix command.  */
       std::string alias_prefix_string (argv_to_string (alias_argv,
 						       alias_argc - 1));
-      std::string command_prefix_string (argv_to_string (alias_argv,
+      std::string command_prefix_string (argv_to_string (command_argv.get (),
 							 command_argc - 1));
       alias_prefix = alias_prefix_string.c_str ();
       command_prefix = command_prefix_string.c_str ();