[C] Fix compliance to C.

Initializing struct return values by initialization lists is not allowed in C.

Author: denizevrenci

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/c/CGenerator.java

@@ -555,9 +555,11 @@ private CharSequence generateVarData(
  " if (!%5$s_set_sbe_position(\n" +
  " codec, %5$s_sbe_position(codec) + %3$d + length_field_value))\n" +
  " {\n" +
- " return {NULL, 0};\n" +
+ " struct %6$s_string_view ret = {NULL, 0};\n" +
+ " return ret;\n" +
  " }\n" +
- " return {field_ptr, length_field_value};\n" +
+ " struct %6$s_string_view ret = {field_ptr, length_field_value};\n" +
+ " return ret;\n" +
  "}\n",
  propertyName,
  generateStringViewNotPresentCondition(token.version()),