Handle cut-off UTF-8 sequences when reading files

Read additional bytes from the file to complete the UTF-8 sequence so the bytes
in it don't get converted to U+FFFD replacement characters.

src/jv_unicode.c

@@ -3,6 +3,29 @@
 #include "jv_unicode.h"
 #include "jv_utf8_tables.h"
 
+// jvp_utf8_backtrack returns the beginning of the last codepoint in the
+// string, assuming that start is the last byte in the string.
+// If the last codepoint is incomplete, returns the number of missing bytes via
+// *missing_bytes.  If there are no leading bytes or an invalid byte is
+// encountered, NULL is returned and *missing_bytes is not altered.
+const char* jvp_utf8_backtrack(const char* start, const char* min, int *missing_bytes) {
+  assert(min < start);
+  if (min == start) {
+    return min;
+  }
+  int length = 0;
+  int seen = 1;
+  while (start >= min && (length = utf8_coding_length[(unsigned char)*start]) == UTF8_CONTINUATION_BYTE) {
+    start--;
+    seen++;
+  }
+  if (length == 0 || length == UTF8_CONTINUATION_BYTE || length - seen < 0) {
+    return NULL;
+  }
+  if (missing_bytes) *missing_bytes = length - seen;
+  return start;
+}
+
 const char* jvp_utf8_next(const char* in, const char* end, int* codepoint_ret) {
   assert(in <= end);
   if (in == end) {