vvp: Only ignore leading null-bytes when reading as string through VPI

Currently when reading a number literal through the VPI API as a
vpiStringVal all null-bytes in the literal get ignored. This behavior is
different from when reading a signal through the VPI API as a vpiStringVal.
The latter will only ignore leading null-bytes and replace other null-bytes
with a space. E.g. the following two will print different values.

```
$display("%s", "a\000b"); // -> " ab"
reg [23:0] x = "a\000b";
$display("%s", x); // -> "a b"
```

For consistency modify the number literal formatting code so that it has
the same behavior as the signal value formatting code and only replaces
leading null-bytes.

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

vvp/vpi_priv.cc

@@ -635,6 +635,8 @@ static void vec4_get_value_string(const vvp_vector4_t&word_val, unsigned width,
 
 	    if (char_val != 0)
 		  *cp++ = char_val;
+	    else if (cp != rbuf)
+		  *cp++ = ' ';
       }
 
       for (unsigned idx = 0 ;  idx < nchar ;  idx += 1) {
@@ -645,8 +647,13 @@ static void vec4_get_value_string(const vvp_vector4_t&word_val, unsigned width,
 		  if (val == BIT4_1)
 			char_val |= 1 << bdx;
 	    }
+	    // Ignore leading null-bytes and replace other null-bytes with space.
+	    // The LRM is not entirely clear on how null bytes should be handled.
+	    // This is the implementation chosen for iverilog.
 	    if (char_val != 0)
 		  *cp++ = char_val;
+	    else if (cp != rbuf)
+		  *cp++ = ' ';
       }
 
       *cp = 0;