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

1 files changed, 3 insertions, 3 deletions

diff --git a/signal.c b/signal.c
index 9047811397..ee707ab547 100644
--- a/signal.c
+++ b/signal.c
@@ -197,8 +197,8 @@ static const struct signals {
 #endif
 };
 
-static const char signame_prefix[3] = "SIG";
-static const int signame_prefix_len = (int)sizeof(signame_prefix);
+static const char signame_prefix[] = "SIG";
+static const int signame_prefix_len = 3;
 
 static int
 signm2signo(VALUE *sig_ptr, int negative, int exit, int *prefix_ptr)
@@ -236,7 +236,7 @@ signm2signo(VALUE *sig_ptr, int negative, int exit, int *prefix_ptr)
 	negative = 0;
     }
     if (len >= prefix + signame_prefix_len) {
-	if (memcmp(nm + prefix, signame_prefix, sizeof(signame_prefix)) == 0)
+	if (memcmp(nm + prefix, signame_prefix, signame_prefix_len) == 0)
 	    prefix += signame_prefix_len;
     }
     if (len <= (long)prefix) {