Fix valgrind error from gdb.decode_line

PR symtab/12535 points out that gdb.decode_line("") will cause a
valgrind report.

I think the empty linespec does not really make sense.  So, this patch
changes gdb.decode_line to treat a whitespace-only linespec the same
as a non-existing argument.

gdb/ChangeLog
2020-01-14  Tom Tromey  <tom@tromey.com>

	PR symtab/12535:
	* python/python.c (gdbpy_decode_line): Treat empty string the same
	as no argument.

gdb/testsuite/ChangeLog
2020-01-14  Tom Tromey  <tom@tromey.com>

	PR symtab/12535:
	* gdb.python/python.exp: Test decode_line with empty string
	argument.

Change-Id: I1d95812b4b7a21d69a3e9afd05b9e3141a931897

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2020-01-14  Tom Tromey  <tom@tromey.com>
+
+	PR symtab/12535:
+	* python/python.c (gdbpy_decode_line): Treat empty string the same
+	as no argument.
+
 2020-01-14  Tom Tromey  <tom@tromey.com>
 
 	* Makefile.in (CLIBS): Remove second use of $(LIBIBERTY).

gdb/python/python.c

@@ -810,6 +810,15 @@ gdbpy_decode_line (PyObject *self, PyObject *args)
   if (! PyArg_ParseTuple (args, "|s", &arg))
     return NULL;
 
+  /* Treat a string consisting of just whitespace the same as
+     NULL.  */
+  if (arg != NULL)
+    {
+      arg = skip_spaces (arg);
+      if (*arg == '\0')
+	arg = NULL;
+    }
+
   if (arg != NULL)
     location = string_to_event_location_basic (&arg, python_language,
 					       symbol_name_match_type::WILD);

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-01-14  Tom Tromey  <tom@tromey.com>
+
+	PR symtab/12535:
+	* gdb.python/python.exp: Test decode_line with empty string
+	argument.
+
 2020-01-14  Bernd Edlinger  <bernd.edlinger@hotmail.de>
 
 	* gdb.base/skip-inline.exp: Extend test.

gdb/testsuite/gdb.python/python.exp

@@ -234,6 +234,10 @@ gdb_test "python print (len(symtab))" "2" "test decode_line current location"
 gdb_test "python print (symtab\[0\])" "None" "test decode_line expression parse"
 gdb_test "python print (len(symtab\[1\]))" "1" "test decode_line current location"
 
+# Test that decode_line with an empty string argument does not crash.
+gdb_py_test_silent_cmd "python symtab2 = gdb.decode_line('')" \
+    "test decode_line with empty string" 1
+
 if { [is_remote host] } {
     set python_c [string_to_regexp "python.c"]
 } else {