main: consider an empty path component when combining a path and file…

… name

combinePathAndFile takes two C strings.
The first one PATH was used in an expression: path [strlen(path) - 1] .
If PATH is an empty, this can cause a buffer overrun.

This change adds a guard-condition for such processing.

Inspired from the comment submitted by @itchyny in universal-ctags#2384.

main/routines.c

@@ -714,14 +714,18 @@ extern char *combinePathAndFile (
 	const char *const path, const char *const file)
 {
 	vString *const filePath = vStringNew ();
-	const int lastChar = path [strlen (path) - 1];
-	bool terminated = isPathSeparator (lastChar);
+	size_t len = strlen (path);
 
-	vStringCopyS (filePath, path);
-	if (! terminated)
-		vStringPut (filePath, OUTPUT_PATH_SEPARATOR);
-	vStringCatS (filePath, file);
+	if (len)
+	{
+		const int lastChar = path [len - 1];
+		bool terminated = isPathSeparator (lastChar);
+		vStringCopyS (filePath, path);
+		if (! terminated)
+			vStringPut (filePath, OUTPUT_PATH_SEPARATOR);
+	}
 
+	vStringCatS (filePath, file);
 	return vStringDeleteUnwrap (filePath);
 }