clear implicit conversion warnings

reported by clang-11 -fsanitize=integer

mkvparser/mkvparser.cc:301:31: runtime error: implicit conversion from
type 'signed char' of value -128 (8-bit, signed) to type 'unsigned long
long' changed the value to 18446744073709551488 (64-bit, unsigned)

m2ts/webm2pes.cc:71:13: runtime error: implicit conversion from type
'unsigned int' of value 1260 (32-bit, unsigned) to type 'std::uint8_t'
(aka 'unsigned char') changed the value to 236 (8-bit, unsigned)

Change-Id: If3d6f17edb7a0d79a944a4dd0919878cf524460b
diff --git a/m2ts/webm2pes.cc b/m2ts/webm2pes.cc
index fc4b314..afa8a6b 100644
--- a/m2ts/webm2pes.cc
+++ b/m2ts/webm2pes.cc
@@ -68,7 +68,7 @@
   // Top 8 bits of second PTS chunk.
   buffer[3] |= (pts3 >> 7) & 0xff;
   // bottom 7 bits of second PTS chunk.
-  buffer[4] |= (pts3 << 1);
+  buffer[4] |= (pts3 << 1) & 0xff;
   // Marker.
   buffer[4] |= 1;
 
diff --git a/mkvparser/mkvparser.cc b/mkvparser/mkvparser.cc
index de8884b..fea1124 100644
--- a/mkvparser/mkvparser.cc
+++ b/mkvparser/mkvparser.cc
@@ -298,7 +298,7 @@
   if (status < 0)
     return status;
 
-  unsigned long long result = first_byte;
+  unsigned long long result = static_cast<unsigned long long>(first_byte);
   ++pos;
 
   for (long i = 1; i < size; ++i) {