tools: kconfig2html: Handle blank input lines

read_line() removes a trailing newline and then checks g_line[len - 1]
for a line-continuation backslash. A blank line leaves len at zero, so
the continuation check reads before g_line.

Return the empty line to the caller before checking for a continuation.
kconfig_line() already skips empty lines, so parser behavior is
unchanged.

Signed-off-by: Old-Ding <ai.neo.ae86@gmail.com>

tools/kconfig2html.c

@@ -702,6 +702,12 @@ static char *read_line(FILE *stream)
           g_line[len] = '\0';
         }
 
+      if (len == 0)
+        {
+          g_lnptr = g_line;
+          return g_line;
+        }
+
       /* Does this continue on the next line?  Note that this check
        * could erroneoulsy combine two lines if a comment line ends with
        * a line continuation... Don't do that!