"enable count" user input error handling (PR gdb/15678)

Typing "enable count" by itself crashes GDB. Also, if you omit the
breakpoint number/range, the error message is not very clear:

(gdb) enable count 2
warning: bad breakpoint number at or near ''
(gdb) enable count
Segmentation fault (core dumped)

With this patch, the error messages are slightly more helpful:

(gdb) enable count 2
Argument required (one or more breakpoint numbers).
(gdb) enable count
Argument required (hit count).

gdb/ChangeLog:

	PR gdb/15678
	* breakpoint.c (map_breakpoint_numbers): Check for empty args
	string.
	(enable_count_command): Check args for NULL value.

gdb/testsuite/ChangeLog:

	PR gdb/15678
	* gdb.base/ena-dis-br.exp: Test "enable count" for bad user input.

1 files changed, 7 insertions, 2 deletions

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index af0c2cf..2804453 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -14880,7 +14880,7 @@ map_breakpoint_numbers (char *args, void (*function) (struct breakpoint *,
   int match;
   struct get_number_or_range_state state;
 
-  if (args == 0)
+  if (args == 0 || *args == '\0')
     error_no_arg (_("one or more breakpoint numbers"));
 
   init_number_or_range (&state, args);
@@ -15217,7 +15217,12 @@ do_map_enable_count_breakpoint (struct breakpoint *bpt, void *countptr)
 static void
 enable_count_command (char *args, int from_tty)
 {
-  int count = get_number (&args);
+  int count;
+
+  if (args == NULL)
+    error_no_arg (_("hit count"));
+
+  count = get_number (&args);
 
   map_breakpoint_numbers (args, do_map_enable_count_breakpoint, &count);
 }