perf script python: Silence -Werror=maybe-uninitialized on gcc 5.3.0

Sounds like a compiler bug, but to silence it, initialize those
variables to NULL.

Noticed on:

Target: x86_64-alpine-linux-musl
Configured with: /home/buildozer/aports/main/gcc/src/gcc-5.3.0/configure
--prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info
--build=x86_64-alpine-linux-musl --host=x86_64-alpine-linux-musl
--target=x86_64-alpine-linux-musl --with-pkgversion='Alpine 5.3.0'
--enable-checking=release --disable-fixed-point --disable-libstdcxx-pch
--disable-multilib --disable-nls --disable-werror --disable-symvers
--enable-__cxa_atexit --enable-esp --enable-cloog-backend
--enable-languages=c,c++,objc,java,fortran,ada --disable-libssp
--disable-libmudflap --disable-libsanitizer --enable-shared
--enable-threads --enable-tls --with-system-zlib
Thread model: posix
gcc version 5.3.0 (Alpine 5.3.0)

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-zyvsjvbl45o7hzcuz78wu2xi@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/scripting-engines/trace-event-python.c

@@ -273,7 +273,7 @@ static PyObject *get_field_numeric_entry(struct event_format *event,
 		struct format_field *field, void *data)
 {
 	bool is_array = field->flags & FIELD_IS_ARRAY;
-	PyObject *obj, *list = NULL;
+	PyObject *obj = NULL, *list = NULL;
 	unsigned long long val;
 	unsigned int item_size, n_items, i;
 
@@ -392,7 +392,7 @@ static void python_process_tracepoint(struct perf_sample *sample,
 				      struct addr_location *al)
 {
 	struct event_format *event = evsel->tp_format;
-	PyObject *handler, *context, *t, *obj, *callchain;
+	PyObject *handler, *context, *t, *obj = NULL, *callchain;
 	PyObject *dict = NULL;
 	static char handler_name[256];
 	struct format_field *field;