systemf(): fix improper handling of WEXITSTATUS()

The call to system() may return -1, e.g. if fork() fails or return
status from execl() cannot be read.  In these cases we should end
immediately and not call WIFEXITED() et al.

Also, WEXITSTATUS() should only be called if WEXITED() returns true.

For details, see system(3) and wait(2).

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/systemf.c

@@ -43,8 +43,9 @@ int systemf(const char *fmt, ...)
 	va_end(ap);
 
 	status = system(cmd);
+	if (status == -1)
+		return -1;
 
-	rc = WEXITSTATUS(status);
 	if (!WIFEXITED(status)) {
 		if (WIFSIGNALED(status) &&
 		    (WTERMSIG(status) == SIGINT ||
@@ -59,7 +60,8 @@ int systemf(const char *fmt, ...)
 			 */
 			rc = 1;
 		}
-	}
+	} else
+		rc = WEXITSTATUS(status);
 
 	return rc;
 }