Stackless issue python#140: Fix compiler warnings

Fix warnings emitted by gcc 7.2.
- ceval.c: '*' in boolean context, suggest '&&' instead.
- stacklessmodule.c: a misleading indentation

(cherry picked from commit c51072e)

Author: Anselm Kruis

Python/ceval.c

@@ -3957,7 +3957,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
     /* Set f->f_lasti to the instruction before the current one or to the
      * first instruction (-1). See "f->f_lasti refers to ..." above.
      */
-    f->f_lasti = INSTR_OFFSET() ?
+    f->f_lasti = INSTR_OFFSET() != 0 ?
             assert(INSTR_OFFSET() >= sizeof(_Py_CODEUNIT)),
             (int)(INSTR_OFFSET() - sizeof(_Py_CODEUNIT)) : -1;
     if (SLP_PEEK_NEXT_FRAME(tstate)->f_back != f)
@@ -4008,7 +4008,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
     /* Set f->f_lasti to the instruction before the current one or to the
      * first instruction (-1). See "f->f_lasti refers to ..." above.
      */
-    f->f_lasti = INSTR_OFFSET() ?
+    f->f_lasti = INSTR_OFFSET() != 0 ?
             assert(INSTR_OFFSET() >= sizeof(_Py_CODEUNIT)),
             (int)(INSTR_OFFSET() - sizeof(_Py_CODEUNIT)) : -1;
     return (PyObject *) Py_UnwindToken;

Stackless/module/stacklessmodule.c

@@ -871,10 +871,9 @@ test_cframe(PyObject *self, PyObject *args, PyObject *kwds)
     if (!PyArg_ParseTupleAndKeywords(args, kwds, "l|l:test_cframe",
                                      argnames, &switches, &extra))
         return NULL;
-        if (extra < 0 || extra > STACK_MAX_USEFUL)
-            VALUE_ERROR(
-                "test_cframe: words are limited by 0 and " \
-                STACK_MAX_USESTR, NULL);
+    if (extra < 0 || extra > STACK_MAX_USEFUL)
+        VALUE_ERROR("test_cframe: words are limited by 0 and " \
+                    STACK_MAX_USESTR, NULL);
     if (extra > 0)
         alloca(extra*sizeof(PyObject*));
     Py_INCREF(ret);