Patch by Ian Good for MDEV-4319: mysqlbinlog output ambiguous escaping

The output of mysqlbinlog (with "-v --base64-output=DECODE-ROWS" flags) can not always be read or parsed correctly
when string columns contain single-quotes or backslash characters.

The fix for this bug is to escape single-quote and backslash characters on output, so that the result is both more
readable and more easily parse-able.

Note that this is just for comments, so it doesn't affect the replication.

sql/log_event.cc:
  Escape \ and ' properly for mysqlbin user comments.

sql/log_event.cc

@@ -1804,6 +1804,7 @@ void Log_event::print_header(IO_CACHE* file,
 /**
   Prints a quoted string to io cache.
   Control characters are displayed as hex sequence, e.g. \x00
+  Single-quote and backslash characters are escaped with a \
   
   @param[in] file              IO cache
   @param[in] prt               Pointer to string
@@ -1819,6 +1820,10 @@ my_b_write_quoted(IO_CACHE *file, const uchar *ptr, uint length)
   {
     if (*s > 0x1F)
       my_b_write(file, s, 1);
+    else if (*s == '\'')
+      my_b_write(file, "\\'", 2);
+    else if (*s == '\\')
+      my_b_write(file, "\\\\", 2);
     else
     {
       uchar hex[10];