Remove NOT_REACHED from Layouter::GetCharPosition

Return begin/end value for unknown code point index or out of range input

See: #596, #598, OpenTTD/OpenTTD#11291

src/gfx_layout.cpp

@@ -230,10 +230,13 @@ Point Layouter::GetCharPosition(std::string_view::const_iterator ch) const
 	const auto &line = this->front();
 
 	/* Pointer to the end-of-string marker? Return total line width. */
-	if (ch == this->string.end()) {
+	if (ch >= this->string.end()) {
 		Point p = { line->GetWidth(), 0 };
 		return p;
 	}
+	if (ch < this->string.begin()) {
+		return { 0, 0 };
+	}
 
 	/* Find the code point index which corresponds to the char
 	 * pointer into our UTF-8 source string. */
@@ -264,7 +267,8 @@ Point Layouter::GetCharPosition(std::string_view::const_iterator ch) const
 		}
 	}
 
-	NOT_REACHED();
+	/* Code point index not found, just give up */
+	return { 0, 0 };
 }
 
 /**