Fix null pointer warning for string argument to parse_struct in registry code

[mgduda]

This merge fixes a null pointer warning for a string argument to parse_struct
in the registry code.

The fifth argument to the parse_struct function in the gen_inc code is
a const char*. The call to parse_struct previously passed a C null character,
'\0', which evaluates to zero and is therefore interpreted as a null pointer.
It was likely intended to pass an empty C string, i.e., a string consisting
of a single null character, in the call to parse_struct, using "\0" rather
than '\0'. This was picked up by the XL C compiler:

gen_inc.c:2531:52: warning: expression which evaluates to zero treated as a null pointer constant of type 'const char *'
[-Wnon-literal-null-conversion]
                err = parse_struct(fd, registry, structs_xml, 0, '\0', corename);
                                                                 ^~~~

This warning turns out to be harmless, as the fifth argument to parse_struct
is only used when the fourth argument is non-zero.

[mgduda]

I think it should be reasonable to test PRs #196 #197 #198 and #199 together.

[mark-petersen]

Visual check, passes ocean nightly regression suite.