Prevent compilation with buggy libcurl library

Closes #34

configure.ac

@@ -1,5 +1,5 @@
 # cmusfm - configure.ac
-# Copyright (c) 2014-2018 Arkadiusz Bokowy
+# Copyright (c) 2014-2020 Arkadiusz Bokowy
 
 AC_PREREQ([2.59])
 AC_INIT([cmusfm], [1.0.0], [[email protected]])
@@ -42,6 +42,14 @@ PKG_CHECK_MODULES([LIBCRYPTO], [libcrypto], [
 ], [#skip])
 
 PKG_CHECK_MODULES([LIBCURL], [libcurl])
+if $PKG_CONFIG --exact-version=7.71.0 libcurl; then
+	AC_MSG_ERROR([libcurl == 7.71.0
+
+Because of curl_easy_escape() bug present in the curl library version 7.71.0,
+cmusfm will not work correctly. Please, upgrade or downgrade the curl library.
+
+Related curl bug report: https://github.com/curl/curl/issues/5601])
+fi
 
 # support for desktop notifications
 AC_ARG_ENABLE([libnotify],

src/libscrobbler2.c

@@ -1,6 +1,6 @@
 /*
  * libscrobbler2.c
- * Copyright (c) 2011-2018 Arkadiusz Bokowy
+ * Copyright (c) 2011-2020 Arkadiusz Bokowy
  *
  * This file is a part of cmusfm.
  *
@@ -58,8 +58,11 @@ static size_t sb_curl_write_callback(char *ptr, size_t size, size_t nmemb,
 
 	/* XXX: Passing a zero bytes data to this callback is not en error,
 	 *      however memory allocation fail is. */
-	if (!size || (rd->data = realloc(rd->data, rd->size + size + 1)) == NULL)
+	void *tmp = rd->data;
+	if (!size || (rd->data = realloc(rd->data, rd->size + size + 1)) == NULL) {
+		free(tmp);
 		return 0;
+	}
 
 	memcpy(&rd->data[rd->size], ptr, size);
 	rd->size += size;