signal.c: make signame_prefix a nul-terminated string

gcc 9.1.0 warns this.

```
signal.c:258:46: warning: '%.*s' directive argument is not a nul-terminated string [-Wformat-overflow=]
  258 |  rb_raise(rb_eArgError, "unsupported signal `%.*s%"PRIsVALUE"'",
      |                                              ^~~~
  259 |    prefix, signame_prefix, vsig);
      |            ~~~~~~~~~~~~~~
signal.c:200:19: note: referenced argument declared here
  200 | static const char signame_prefix[3] = "SIG";
      |                   ^~~~~~~~~~~~~~
```
https://rubyci.org/logs/rubyci.s3.amazonaws.com/ubuntu/ruby-master/log/20190801T033009Z.log.html.gz

0 files changed, 0 insertions, 0 deletions