compile.c: prevent out-of-bound initialization of coverage counters

The coverage counters is initialized with `counter[lineno - 1] = 0`,
but lineno may be 0, which led to write access for index -1.
[ruby-core:90085] [Bug#15346]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66025 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

compile.c

@@ -2027,7 +2027,9 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *const anchor)
                     if (ISEQ_LINE_COVERAGE(iseq) && (events & RUBY_EVENT_COVERAGE_LINE) &&
                         !(rb_get_coverage_mode() & COVERAGE_TARGET_ONESHOT_LINES)) {
                         int line = iobj->insn_info.line_no;
-                        RARRAY_ASET(ISEQ_LINE_COVERAGE(iseq), line - 1, INT2FIX(0));
+			if (line >= 1) {
+			    RARRAY_ASET(ISEQ_LINE_COVERAGE(iseq), line - 1, INT2FIX(0));
+			}
                     }
                     if (ISEQ_BRANCH_COVERAGE(iseq) && (events & RUBY_EVENT_COVERAGE_BRANCH)) {
                         while (RARRAY_LEN(ISEQ_PC2BRANCHINDEX(iseq)) <= code_index) {