minor cleanings to avoid warnings from the clang static analyzer

Anyway it's just about some dead-store warnings :

    src/mongoose.c:1159:5: warning: Value stored to 'p' is never read
        p = (char *) prog;
        ^   ~~~~~~~~~~~~~
    src/mongoose.c:1517:3: warning: Value stored to 'res' is never read
        res = -1;
        ^     ~~
    src/mongoose.c:1731:3: warning: Value stored to 'a' is never read
        a = b = c = d = e = block[0].l[0];
        ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    3 warnings generated.