Fix out-of-bounds error in strsignal

tests/string/strsignal.c

@@ -0,0 +1,28 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <signal.h>
+
+#include "test_helpers.h"
+
+int main(void) {
+    puts("# strsignal #");
+    char *x = strsignal(SIGHUP);
+    int res;
+    if (strcmp(x, "Hangup")) {
+        printf("Incorrect strsignal (1), found: .%s.\n", x);
+        exit(EXIT_FAILURE);
+    }
+    x = strsignal(0); 
+    if (strcmp(x, "Unknown signal")) {
+        printf("Incorrect strsignal (2), found: .%s.\n", x);
+        exit(EXIT_FAILURE);
+    }
+    x = strsignal(100); 
+    if (strcmp(x, "Unknown signal")) {
+        printf("Incorrect strsignal (3), found: .%s.\n", x);
+        exit(EXIT_FAILURE);
+    }
+
+
+}