kill: Fix free() with bad pointer on SIG-prefixed signal-name

kill -l SIGHUP (or any other signal-name prefixed with "SIG")
would cause free() to be called with a bad pointer instead of
a pointer to what was allocated. Fix this and add test-case.

lib/signals.c

@@ -263,7 +263,7 @@ char *strtosig(const char *restrict s)
             }
         }
     }
-    free(p);
+    free(copy);
     return converted;
 }
 

testsuite/kill.test/kill.exp

@@ -24,6 +24,10 @@ set test "kill convert signal name to number"
 spawn  $kill -lHUP
 expect_pass "$test" "^1\\s*"
 
+set test "kill convert SIG-prefixed signal name to number"
+spawn  $kill -lSIGHUP
+expect_pass "$test" "^1\\s*$"
+
 set test "kill convert signal number to name"
 spawn  $kill -l 1
 expect_pass "$test" "^HUP\\s*"