vvp: Remove null-bytes when converting string literals to SV strings

The `%pushi/str` and `%concati/str` instructions should remove null-bytes
from the string literal when converting it to a string. This is defined in
section 6.16 ("String data type") of the LRM (1800-2017).

This is already handled correctly when converting a vector from the stack
to a SV string, just not when converting a string literal to SV string.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

vvp/vthread.cc

@@ -399,7 +399,11 @@ static string filter_string(const char*text)
 		  cnt -= 1;
 		  ptr += 1;
 	    }
-	    tmp[dst++] = byte;
+
+	    // null-bytes are supposed to be removed when assigning a string
+	    // literal to a string.
+	    if (byte != '\0')
+		  tmp[dst++] = byte;
 
 	    // After the while loop above, the ptr points to the next character,
 	    // but the for-loop condition is assuming that ptr points to the last