error: Add error code for sig-related diagnostics

This commit adds an error code for the diagnostic code used for
diagnostic messages following unexpected termination due to signals..

(cherry picked from commit 501c870a2c)

src/util-error.c

@@ -379,6 +379,7 @@ const char * SCErrorToString(SCError err)
         CASE_CODE(SC_ERR_RULE_INVALID_UTF8);
         CASE_CODE(SC_WARN_CHOWN);
         CASE_CODE(SC_ERR_HASH_ADD);
+        CASE_CODE(SC_ERR_SIGNAL);
 
         CASE_CODE (SC_ERR_MAX);
     }

src/util-error.h

@@ -369,6 +369,7 @@ typedef enum {
     SC_ERR_RULE_INVALID_UTF8,
     SC_WARN_CHOWN,
     SC_ERR_HASH_ADD,
+    SC_ERR_SIGNAL,
 
     SC_ERR_MAX
 } SCError;