Fix a stringop-truncation warning reported by gcc 10

In function ‘strncpy’,
    inlined from ‘add_classid_to_signatureclass’ at /home/travis/build/nickbroon/vermont/src/modules/analysis/fpsigmatcher/Signature.c:204:2:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 50 equals destination size [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/modules/analysis/fpsigmatcher/Signature.c

@@ -201,7 +201,7 @@ void add_classid_to_signatureclass(struct SignatureClass * class, char * new_mem
 	// I think that a class id is not added to a signature, but a signature id to a class.
 	class->member_IDs = realloc(class->member_IDs, sizeof(char *) * (class->numOfMembers + 1));
 	class->member_IDs[class->numOfMembers] = malloc(sizeof(char) * ID_STRING_SIZE);
-	strncpy(class->member_IDs[class->numOfMembers], new_memberID, ID_STRING_SIZE);
+	strncpy(class->member_IDs[class->numOfMembers], new_memberID, ID_STRING_SIZE-1);
 	class->member_IDs[class->numOfMembers][ID_STRING_SIZE-1] = '\0';
 	class->numOfMembers++;
 }