pgrep: Remove memory leak

This is part of !118 where @tt.rantala found a memory leak.
The other part of !118 may come later if the performance change
is significant.

References:
 procps-ng/procps!118

pgrep.c

@@ -716,6 +716,11 @@ static struct el * select_procs (int *num)
         free(cmdsearch);
         free(cmdoutput);
 
+	if (preg) {
+		regfree(preg);
+		free(preg);
+	}
+
 	return list;
 }