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

@@ -663,6 +663,11 @@ static struct el * select_procs (int *num)
     free(cmdsearch);
     free(cmdoutput);
 
+    if (preg) {
+        regfree(preg);
+        free(preg);
+    }
+
     *num = matches;
 
     if ((!matches) && (!opt_full) && opt_pattern && (strlen(opt_pattern) > 15))