bit_lib: don't read past the buffer when the bits fit one byte (#4424)

bit_lib_get_bits reads data[position/8 + 1] whenever position isn't
byte-aligned, even when all the requested bits live in the current byte.
When position/8 is the last byte of the buffer that's a one-byte
over-read. The extra byte only contributes bits that get shifted back
out, so the return value is unchanged and optimized builds often drop
the load, but at -O0 AddressSanitizer flags it and the access is still
out of bounds.

Skip the next-byte read when shift + length <= 8. This is the same fix
that already landed in fbtng-corelibs; lib/bit_lib here is an independent
copy that never picked it up, so the TODO FL-3534 comment is still here.

Signed-off-by: Cole Munz <colemunz@gmail.com>

Author: munzzyy

lib/bit_lib/bit_lib.c

@@ -37,8 +37,10 @@ uint8_t bit_lib_get_bits(const uint8_t* data, size_t position, uint8_t length) {
     uint8_t shift = position % 8;
     if(shift == 0) {
         return data[position / 8] >> (8 - length);
+    } else if(shift + length <= 8) {
+        // Requested bits fit in the current byte, don't read the next one
+        return (uint8_t)(data[position / 8] << shift) >> (8 - length);
     } else {
-        // TODO FL-3534: fix read out of bounds
         uint8_t value = (data[position / 8] << (shift));
         value |= data[position / 8 + 1] >> (8 - shift);
         value = value >> (8 - length);