Fix segmentation fault when the value of some external variable can't be converted to string.

See: https://github.com/VirusTotal/yara/issues/990

tests.py

@@ -813,6 +813,10 @@ class TestYara(unittest.TestCase):
         r = yara.compile(source='rule test { condition: ext_str matches /ssi$/ }', externals={'ext_str': 'mississippi'})
         self.assertFalse(r.match(data='dummy'))
 
+        self.assertRaises(UnicodeEncodeError, yara.compile,
+            source="rule test { condition: true}",
+            externals={'foo': u'\u6765\u6613\u7f51\u7edc\u79d1' })
+
     def testCallbackAll(self):
         global rule_data
         rule_data = []

yara-python.c

@@ -1002,10 +1002,13 @@ int process_compile_externals(
     }
     else if (PY_STRING_CHECK(value))
     {
+      char* str = PY_STRING_TO_C(value);
+
+      if (str == NULL)
+        return ERROR_INVALID_ARGUMENT;
+
       result = yr_compiler_define_string_variable(
-          compiler,
-          identifier,
-          PY_STRING_TO_C(value));
+          compiler, identifier, str);
     }
     else
     {
@@ -1069,10 +1072,13 @@ int process_match_externals(
     }
     else if (PY_STRING_CHECK(value))
     {
+      char* str = PY_STRING_TO_C(value);
+
+      if (str == NULL)
+        return ERROR_INVALID_ARGUMENT;
+
       result = yr_rules_define_string_variable(
-          rules,
-          identifier,
-          PY_STRING_TO_C(value));
+          rules, identifier, str);
     }
     else
     {