Fix warnings in C code

[lfborjas]

csrc/configurable_sweephe4.c: In function ‘eph4_posit’:

csrc/configurable_sweephe4.c:555:21: error:
     warning: ‘%s’ directive writing 5 bytes into a region of size between 1 and 256 [-Wformat-overflow=]
      555 |       sprintf(s, "%s%sM%d", ephe4d.ephe4path, EP4_FILE, -filenr);
          |                     ^~
    |
555 |       sprintf(s, "%s%sM%d", ephe4d.ephe4path, EP4_FILE, -filenr);
    |                     ^

csrc/configurable_sweephe4.c:555:18: error:
     note: directive argument in the range [1, 214749]
      555 |       sprintf(s, "%s%sM%d", ephe4d.ephe4path, EP4_FILE, -filenr);
          |                  ^~~~~~~~~
    |
555 |       sprintf(s, "%s%sM%d", ephe4d.ephe4path, EP4_FILE, -filenr);
    |                  ^
In file included from /usr/include/stdio.h:867,
                 from csrc/sweodef.h:169,
                 from csrc/swephexp.h:82,

                 from csrc/configurable_sweephe4.c:60:0: error: 

/usr/include/x86_64-linux-gnu/bits/stdio2.h:36:10: error:
     note: ‘__builtin___sprintf_chk’ output between 8 and 268 bytes into a destination of size 256
       36 |   return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
    |                     ^

csrc/configurable_sweephe4.c:553:18: error:
     note: directive argument in the range [0, 214748]
      553 |       sprintf(s, "%s%s%d", ephe4d.ephe4path, EP4_FILE, filenr);
          |                  ^~~~~~~~
    |
553 |       sprintf(s, "%s%s%d", ephe4d.ephe4path, EP4_FILE, filenr);
    |                  ^
In file included from /usr/include/stdio.h:867,
                 from csrc/sweodef.h:169,
                 from csrc/swephexp.h:82,

                 from csrc/configurable_sweephe4.c:60:0: error: 

/usr/include/x86_64-linux-gnu/bits/stdio2.h:36:10: error:
     note: ‘__builtin___sprintf_chk’ output between 7 and 267 bytes into a destination of size 256
       36 |   return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
          |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       37 |       __bos (__s), __fmt, __va_arg_pack ());
          |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   |
36 |   return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
   |          ^
csrc/configurable_sweephe4.c: In function ‘ephe4_write_file’:

csrc/configurable_sweephe4.c:990:61: error:
     warning: ‘%s’ directive output may be truncated writing up to 255 bytes into a region of size 235 [-Wformat-truncation=]
      990 |           snprintf(errtext, AS_MAXCH, "error in swe_calc(): %s", serr);
          |                                                             ^~   ~~~~
    |
990 |           snprintf(errtext, AS_MAXCH, "error in swe_calc(): %s", serr);
    |                                                             ^
In file included from /usr/include/stdio.h:867,
                 from csrc/sweodef.h:169,
                 from csrc/swephexp.h:82,

                 from csrc/configurable_sweephe4.c:60:0: error: 

/usr/include/x86_64-linux-gnu/bits/stdio2.h:67:10: error:
     note: ‘__builtin___snprintf_chk’ output between 22 and 277 bytes into a destination of size 256
       67 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
          |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       68 |        __bos (__s), __fmt, __va_arg_pack ());
          |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   |
67 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
   |          ^