Fix double free crash

gpg.c
Under certain circumstances setting plain_str[len] to 0 might lead to crash
and it does not follow the best practices as well.

This change allows better handling of buffer copying and prevents crash.

ox.c
In this implementation gpg buffer remains untouched.
So it's just a code clean up, not a fix.

src/pgp/gpg.c

@@ -721,10 +721,9 @@ p_gpg_decrypt(const char* const cipher)
     char* plain_str = gpgme_data_release_and_get_mem(plain_data, &len);
     char* result = NULL;
     if (plain_str) {
-        plain_str[len] = 0;
-        result = g_strdup(plain_str);
+        result = g_strndup(plain_str, len);
+        gpgme_free(plain_str);
     }
-    gpgme_free(plain_str);
 
     if (passphrase_attempt) {
         passphrase = strdup(passphrase_attempt);

src/pgp/ox.c

@@ -419,10 +419,12 @@ p_ox_gpg_decrypt(char* base64)
 
     size_t len;
     char* plain_str = gpgme_data_release_and_get_mem(plain, &len);
-    char* result = malloc(len + 1);
-    memcpy(result, plain_str, len);
-    result[len] = '\0';
-    gpgme_free(plain_str);
+    char* result = NULL;
+    if (plain_str) {
+        result = g_strndup(plain_str, len);
+        gpgme_free(plain_str);
+    }
+
     return result;
 }