Fix PR breakpoints/16889: gdb segfaults when printing ASM SDT arguments

This commit fixes PR breakpoints/16889, which is about a bug that
triggers when GDB tries to parse probes whose arguments do not contain
the initial (and optional) "N@" part.  For reference sake, the de
facto format is described here:

  <https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation>

Anyway, this PR actually uncovered two bugs (related) that were
happening while parsing the arguments.  The first one was that the
parser *was* catching *some* arguments that were missing the "N@"
part, but it wasn't correctly setting the argument's type.  This was
causing a NULL pointer being dereferenced, ouch...

The second bug uncovered was that the parser was not catching all of
the cases for a probe which did not provide the "N@" part.  The fix
for that was to simplify the check that the code was making to
identify non-prefixed probes.  The code is simpler and easier to read
now.

I am also providing a testcase for this bug, only for x86_64
architectures.

gdb/
2014-05-02  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR breakpoints/16889
	* stap-probe.c (stap_parse_probe_arguments): Simplify
	check for non-prefixed probes (i.e., probes whose
	arguments do not start with "N@").  Always set the
	argument type to a sane value.

gdb/testsuite/
2014-05-02  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR breakpoints/16889
	* gdb.arch/amd64-stap-optional-prefix.S: New file.
	* gdb.arch/amd64-stap-optional-prefix.exp: Likewise.

1 files changed, 6 insertions, 5 deletions

diff --git a/gdb/stap-probe.c b/gdb/stap-probe.c
index dbe9f31bd6..ef45495bb3 100644
--- a/gdb/stap-probe.c
+++ b/gdb/stap-probe.c
@@ -1098,10 +1098,8 @@ stap_parse_probe_arguments (struct stap_probe *probe, struct gdbarch *gdbarch)
 	 Where `N' can be [+,-][4,8].  This is not mandatory, so
 	 we check it here.  If we don't find it, go to the next
 	 state.  */
-      if ((*cur == '-' && cur[1] != '\0' && cur[2] != '@')
-	  && cur[1] != '@')
-	arg.bitness = STAP_ARG_BITNESS_UNDEFINED;
-      else
+      if ((cur[0] == '-' && isdigit (cur[1]) && cur[2] == '@')
+	  || (isdigit (cur[0]) && cur[1] == '@'))
 	{
 	  if (*cur == '-')
 	    {
@@ -1127,11 +1125,14 @@ stap_parse_probe_arguments (struct stap_probe *probe, struct gdbarch *gdbarch)
 	    }
 
 	  arg.bitness = b;
-	  arg.atype = stap_get_expected_argument_type (gdbarch, b);
 
 	  /* Discard the number and the `@' sign.  */
 	  cur += 2;
 	}
+      else
+	arg.bitness = STAP_ARG_BITNESS_UNDEFINED;
+
+      arg.atype = stap_get_expected_argument_type (gdbarch, arg.bitness);
 
       expr = stap_parse_argument (&cur, arg.atype, gdbarch);