terminal_input_parser: validate UTF8.

Make sure code points parsed are always valid UTF8. Don't assume stdin
is filled with valid data. Check for overlong UTF8 and add some tests.

The fuzzer has reached the following coverage:
- cov    : 204
- ft     : 754
- corp   : 62/12257b
- lim    : 2798
- exec/s : 1748
- rss    : 445Mb
- L      : 155/1946
- MS     : 3

Fixed:https://github.com/ArthurSonzogni/FTXUI/issues/118

src/ftxui/component/terminal_input_parser.cpp

@@ -93,14 +93,78 @@ TerminalInputParser::Output TerminalInputParser::Parse() {
   return ParseUTF8();
 }
 
+// Code point <-> UTF-8 conversion
+//
+// ┏━━━━━━━━┳━━━━━━━━┳━━━━━━━━┳━━━━━━━━┓
+// ┃Byte 1  ┃Byte 2  ┃Byte 3  ┃Byte 4  ┃
+// ┡━━━━━━━━╇━━━━━━━━╇━━━━━━━━╇━━━━━━━━┩
+// │0xxxxxxx│        │        │        │
+// ├────────┼────────┼────────┼────────┤
+// │110xxxxx│10xxxxxx│        │        │
+// ├────────┼────────┼────────┼────────┤
+// │1110xxxx│10xxxxxx│10xxxxxx│        │
+// ├────────┼────────┼────────┼────────┤
+// │11110xxx│10xxxxxx│10xxxxxx│10xxxxxx│
+// └────────┴────────┴────────┴────────┘
+//
+// Then some sequences are illegal if it exist a shorter representation of the
+// same codepoint.
 TerminalInputParser::Output TerminalInputParser::ParseUTF8() {
   unsigned char head = static_cast<unsigned char>(Current());
-  for (int i = 0; i < 3; ++i, head <<= 1) {
-    if ((head & 0b11000000) != 0b11000000)
-      break;
+  unsigned char selector = 0b1000'0000;
+
+  // The non code-point part of the first byte.
+  unsigned char mask = selector;
+
+  // Find the first zero in the first byte.
+  int first_zero = 8;
+  for(int i = 0; i<8; ++i) {
+    mask |= selector;
+    if (head & selector) {
+      selector >>= 1;
+      continue;
+    }
+    first_zero = i;
+    break;
+  }
+
+  // Accumulate the value of the first byte.
+  wchar_t value = head & ~mask;
+
+  // Invalid UTF8, with more than 5 bytes.
+  if (first_zero == 1 || first_zero >= 5)
+    return DROP;
+
+  // Multi byte UTF-8.
+  for (int i = 2; i <= first_zero; ++i) {
     if (!Eat())
       return UNCOMPLETED;
+
+    // Invalid continuation byte.
+    head = static_cast<unsigned char>(Current());
+    if ((head & 0b1100'0000) != 0b1000'0000)
+      return DROP;
+    value <<= 6;
+    value += head & 0b0011'1111;
   }
+
+  // Check for overlong UTF8 encoding.
+  int extra_byte;
+  if (value <= 0b000'0000'0111'1111) {
+    extra_byte = 0;
+  } else if (value <= 0b000'0111'1111'1111) {
+    extra_byte = 1;
+  } else if (value <= 0b1111'1111'1111'1111) {
+    extra_byte = 2;
+  } else if (value <= 0b1'0000'1111'1111'1111'1111) {
+    extra_byte = 3;
+  } else {
+    return DROP;
+  }
+
+  if (extra_byte != position_)
+    return DROP;
+
   return CHARACTER;
 }