Add product string of Hydrometer Sharky 775

According to W4B21021.pdf Appendix A a hex code Fh in the MSD
position signals a negative BCD number.

mbus/mbus-protocol.c

@@ -461,9 +461,9 @@ mbus_data_bcd_encode(unsigned char *bcd_data, size_t bcd_data_size, int value)
     int v0, v1, v2, x1, x2;
     size_t i;
 
-    if (bcd_data && bcd_data_size && (value >= 0))
+    if (bcd_data && bcd_data_size)
     {
-        v2 = value;
+        v2 = abs(value);
 
         for (i = 0; i < bcd_data_size; i++)
         {
@@ -477,6 +477,11 @@ mbus_data_bcd_encode(unsigned char *bcd_data, size_t bcd_data_size, int value)
             bcd_data[bcd_data_size-1-i] = (x2 << 4) | x1;
         }
 
+        if (value < 0)
+        {
+            bcd_data[bcd_data_size-1] |= 0xF0;
+        }
+
         return 0;
     }
 
@@ -498,10 +503,22 @@ mbus_data_bcd_decode(unsigned char *bcd_data, size_t bcd_data_size)
     {
         for (i = bcd_data_size; i > 0; i--)
         {
-            val = (val * 10) + ((bcd_data[i-1]>>4) & 0xF);
+            val = (val * 10);
+
+            if (bcd_data[i-1]>>4 < 0xA)
+            {
+                val += ((bcd_data[i-1]>>4) & 0xF);
+            }
+
             val = (val * 10) + ( bcd_data[i-1] & 0xF);
         }
 
+        // hex code Fh in the MSD position signals a negative BCD number
+        if (bcd_data[bcd_data_size-1]>>4 == 0xF)
+        {
+            val *= -1;
+        }
+
         return val;
     }
 
@@ -1088,6 +1105,9 @@ mbus_data_product_name(mbus_data_variable_header *header)
                 case 0x28:
                     strcpy(buff,"ABB F95 Typ US770");
                     break;
+                case 0x2F:
+                    strcpy(buff,"Hydrometer Sharky 775");
+                    break;
             }
         }
         else if (manufacturer == mbus_manufacturer_id("JAN"))