Plug a memory leak

Addressing this OpenScanHub issue:
2. Defect type: RESOURCE_LEAK
15. openscap-1.4.1/src/OVAL/probes/independent/textfilecontent_probe.c:203:3: alloc_arg: "oscap_pcre_get_substrings" allocates memory that is stored into "substrs".
17. openscap-1.4.1/src/OVAL/probes/independent/textfilecontent_probe.c:209:4: noescape: Resource "substrs" is not freed or pointed-to in "create_item".
28. openscap-1.4.1/src/OVAL/probes/independent/textfilecontent_probe.c:228:2: leaked_storage: Variable "substrs" going out of scope leaks the storage it points to.
   226|   	free(whole_path_with_prefix);
   227|
   228|-> 	return ret;
   229|   }
   230|

src/OVAL/probes/independent/textfilecontent_probe.c

@@ -213,6 +213,7 @@ static int process_file(const char *prefix, const char *path, const char *filena
 
 			for (k = 0; k < substr_cnt; ++k)
 				free(substrs[k]);
+			free(substrs);
 		}
 	}