bug#15307: minor fix to dfa.c

[Aharon Robbins]

The following fix to dfa.c was suggested by a static checking tool.
I'm applying it in the gawk code base.

Basically, it's theoretically possible for len to have run off the end
of the `str' array.

Thanks,

Arnold

diff --git a/dfa.c b/dfa.c
index 8b79eb7..490a075 100644
--- a/dfa.c
+++ b/dfa.c
@@ -1038,7 +1038,8 @@ parse_bracket_exp (void)
                     /* This is in any case an invalid class name.  */
                     str[0] = '\0';
                 }
-              str[len] = '\0';
+              if (len < BRACKET_BUFFER_SIZE)
+                 str[len] = '\0';
 
               /* Fetch bracket.  */
               FETCH_WC (c, wc, _("unbalanced ["));