Fix critical oversight in lexer buffer refilling

Since the lexer buffer wraps, the refilling gets handled in two steps:
First, iff the buffer would wrap, the buffer is refilled until its end.
Then, if more characters are requested, that amount is refilled too.

An important detail is that `read()` may not return as many characters as
requested; for this reason, the first step checks if its `read()` was
"full", and skips the second step otherwise.
This is also where a bug lied.

After a *lot* of trying, I eventually managed to reproduce the bug on an
OpenBSD VM, and after adding a couple of `assert`s in `peekInternal`, this
is what happened, starting at line 724:

0. `lexerState->nbChars` is 0, `lexerState->index` is 19;
1. We end up with `target` = 42, and `writeIndex` = 19;
2. 42 + 19 is greater than `LEXER_BUF_SIZE` (= 42), so the `if` is entered;
3. Within the first `readChars`, **`read` only returns 16 bytes**,
   advancing `writeIndex` to 35 and `target` to 26;
4. Within the second `readChars`, a `read(26)` is issued, overflowing the
   buffer.

The bug should be clear now: **the check at line 750 failed to work!** Why?
Because `readChars` modifies `writeIndex`.
The fix is simply to cache the number of characters expected, and use that.

src/asm/lexer.c

@@ -729,6 +729,8 @@ static int peekInternal(uint8_t distance)
 		ssize_t nbCharsRead = 0, totalCharsRead = 0;
 
 #define readChars(size) do { \
+	/* This buffer overflow made me lose WEEKS of my life. Never again. */ \
+	assert(writeIndex + (size) <= LEXER_BUF_SIZE); \
 	nbCharsRead = read(lexerState->fd, &lexerState->buf[writeIndex], (size)); \
 	if (nbCharsRead == -1) \
 		fatalerror("Error while reading \"%s\": %s\n", lexerState->path, errno); \
@@ -741,9 +743,11 @@ static int peekInternal(uint8_t distance)
 
 		/* If the range to fill passes over the buffer wrapping point, we need two reads */
 		if (writeIndex + target > LEXER_BUF_SIZE) {
-			readChars(LEXER_BUF_SIZE - writeIndex);
+			size_t nbExpectedChars = LEXER_BUF_SIZE - writeIndex;
+
+			readChars(nbExpectedChars);
 			/* If the read was incomplete, don't perform a second read */
-			if (nbCharsRead < LEXER_BUF_SIZE - writeIndex)
+			if (nbCharsRead < nbExpectedChars)
 				target = 0;
 		}
 		if (target != 0)