BR3392776: parser: parse_line -- fix unitialized memory access

Andrew reported that we may access unitialized memory

> SUMMARY: MemorySanitizer: use-of-uninitialized-value nasm/asm/parser.c:982:41 in parse_line

It turns out that in case of malformed data the expression is terminator
itself so we should not "lookup ahead" for next one. Thus test for first
expression initially and if test passes check for terminator.

Reported-by: Andrew Bao <xiaobaozidi@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

Author: cyrillos

asm/parser.c

@@ -979,13 +979,12 @@ insn *parse_line(char *buffer, insn *result)
             /*
              * Process the segment override.
              */
-            if (value[1].type   != 0    ||
-                value->value    != 1    ||
-                !IS_SREG(value->type))
+            if (!IS_SREG(value->type) || value->value != 1 ||
+                value[1].type != 0) {
                 nasm_nonfatal("invalid segment override");
-            else if (result->prefixes[PPS_SEG])
+            } else if (result->prefixes[PPS_SEG]) {
                 nasm_nonfatal("instruction has conflicting segment overrides");
-            else {
+            } else {
                 result->prefixes[PPS_SEG] = value->type;
                 if (IS_FSGS(value->type))
                     op->eaflags |= EAF_FSGS;