Fix #79756: finfo_file crash (FILEINFO_MIME)

If `ctime` or `asctime` return `NULL`, we must not attempt to copy the
buffer, but rather return `NULL` as well.

NEWS

@@ -10,6 +10,9 @@ PHP                                                                        NEWS
   . Fixed bug #79741 (curl_setopt CURLOPT_POSTFIELDS asserts on object with
     declared properties). (Nikita)
 
+- Fileinfo:
+  . Fixed bug #79756 (finfo_file crash (FILEINFO_MIME)). (cmb)
+
 - FTP:
   . Fixed bug #55857 (ftp_size on large files). (cmb)
 

ext/fileinfo/tests/bug79756.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Bug #79756 (finfo_file crash (FILEINFO_MIME))
+--SKIPIF--
+<?php
+if (!extension_loaded('fileinfo')) die('skip fileinfo extension not available');
+?>
+--FILE--
+<?php
+$filename = __DIR__ . '/bug79756.xls';
+$finfo = finfo_open(FILEINFO_MIME);
+$mime = finfo_file($finfo, $filename);
+finfo_close($finfo);
+echo $mime;
+?>
+--EXPECT--
+application/vnd.ms-excel; charset=binary

main/reentrancy.c

@@ -187,11 +187,14 @@ PHPAPI char *php_ctime_r(const time_t *clock, char *buf)
 	local_lock(CTIME_R);
 
 	tmp = ctime(clock);
-	strcpy(buf, tmp);
+	if (tmp) {
+		strcpy(buf, tmp);
+		tmp = buf;
+	}
 
 	local_unlock(CTIME_R);
 
-	return buf;
+	return tmp;
 }
 
 #endif
@@ -205,11 +208,14 @@ PHPAPI char *php_asctime_r(const struct tm *tm, char *buf)
 	local_lock(ASCTIME_R);
 
 	tmp = asctime(tm);
-	strcpy(buf, tmp);
+	if (tmp) {
+		strcpy(buf, tmp);
+		tmp = buf;
+	}
 
 	local_unlock(ASCTIME_R);
 
-	return buf;
+	return tmp;
 }
 
 #endif