Fix/remove comparisons that will always be true

src/core/bytecode.c: In function ‘MVM_bytecode_resolve_annotation’:
src/core/bytecode.c:924:5: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
     if (sfb->num_annotations && offset >= 0 && offset < sfb->bytecode_size) {
     ^

src/core/frame.c: In function ‘MVM_frame_vivify_lexical’:
src/core/frame.c:961:5: warning: comparison is always true due to limited range of data type [-Wtype-limits]
     if (flag != -1 && static_env[effective_idx].o == NULL) {
     ^

src/spesh/facts.c: In function ‘wval_facts’:
src/spesh/facts.c:115:5: warning: comparison is always true due to limited range of data type [-Wtype-limits]
     if (dep >= 0 && dep < cu->body.num_scs) {
     ^

src/strings/ascii.c: In function ‘MVM_string_ascii_decode’:
src/strings/ascii.c:17:9: warning: comparison is always true due to limited range of data type [-Wtype-limits]
         if (ascii[i] <= 127)
         ^