Add error if yield is used outside a generator

The yield statement can only be used in generator functions, which are
marked with an asterix.

Zend/tests/generators/yield_in_normal_function_error.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Yield cannot be used in normal (non-generator) functions
+--FILE--
+<?php
+
+function foo() {
+    yield "Test";
+}
+
+?>
+--EXPECTF--
+Fatal error: The "yield" statement can only be used inside a generator function in %s on line %d

Zend/tests/generators/yield_outside_function_error.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Yield cannot be used outside of functions
+--FILE--
+<?php
+
+yield "Test";
+
+?>
+--EXPECTF--
+Fatal error: The "yield" statement can only be used inside a generator function in %s on line %d

Zend/zend_compile.c

@@ -2660,7 +2660,11 @@ void zend_do_return(znode *expr, int do_end_vparse TSRMLS_DC) /* {{{ */
 
 void zend_do_yield(znode *expr TSRMLS_DC) /* {{{ */
 {
-    /* do nothing for now */
+	if ((CG(active_op_array)->fn_flags & ZEND_ACC_GENERATOR) == 0) {
+		zend_error(E_COMPILE_ERROR, "The \"yield\" statement can only be used inside a generator function");
+	}
+
+	/* do nothing for now */
 }
 /* }}} */