BLI_make_file_string wasn't guaranteed to initialize the resulting path, some parts of the code accounted for this but most not, always initialize the string to "".

source/blender/blenlib/intern/path_util.c

@@ -1257,9 +1257,18 @@ void BLI_make_file_string(const char *relabase, char *string,  const char *dir,
 {
 	int sl;
 
-	if (!string || !dir || !file) return; /* We don't want any NULLs */
-	
-	string[0]= 0; /* ton */
+	if (string) {
+		/* ensure this is always set even if dir/file are NULL */
+		string[0]= '\0';
+
+		if (ELEM(NULL, dir, file)) {
+			return; /* We don't want any NULLs */
+		}
+	}
+	else {
+		return; /* string is NULL, probably shouldnt happen but return anyway */
+	}
+
 
 	/* we first push all slashes into unix mode, just to make sure we don't get
 	   any mess with slashes later on. -jesterKing */

source/blender/blenloader/intern/writefile.c

@@ -2699,7 +2699,6 @@ int BLO_write_file(Main *mainvar, const char *filepath, int write_flags, ReportL
 		}
 	}
 
-	userfilename[0]= '\0'; /* ensure its initialized */
 	BLI_make_file_string(G.main->name, userfilename, BLI_get_folder_create(BLENDER_USER_CONFIG, NULL), BLENDER_STARTUP_FILE);
 	write_user_block= (BLI_path_cmp(filepath, userfilename) == 0);
 

source/blender/editors/render/render_preview.c

@@ -122,8 +122,6 @@ ImBuf* get_brush_icon(Brush *brush)
 				if (!(brush->icon_imbuf)) {
 					folder= BLI_get_folder(BLENDER_DATAFILES, "brushicons");
 
-					path[0]= 0;
-
 					BLI_make_file_string(G.main->name, path, folder, brush->icon_filepath);
 
 					if (path[0])