Fix a compilation warning in command.c relating to kpos shadowing

When building with the gcc -Wshadow option, a warning is generated
in redis_parse_cmd() because the second and third declarations of
the same-named kpos variable share scope.  It's benign but it'd be
great to have clean compilation when using this option.

This commit implements Viktor Söderqvist's refactoring approach as
described in github PR #201.

command.c

@@ -199,6 +199,15 @@ char *redis_parse_bulk(char *p, char *end, char **str, uint32_t *len) {
     return p;
 }
 
+static inline int push_keypos(struct cmd *r, char *arg, uint32_t arglen) {
+    struct keypos *kpos = hiarray_push(r->keys);
+    if (kpos == NULL)
+        return 0;
+    kpos->start = arg;
+    kpos->end = arg + arglen;
+    return 1;
+}
+
 /*
  * Reference: http://redis.io/topics/protocol
  *
@@ -306,11 +315,8 @@ void redis_parse_cmd(struct cmd *r) {
                 /* Keyword found. Now the first key is the next arg. */
                 if ((p = redis_parse_bulk(p, end, &arg, &arglen)) == NULL)
                     goto error;
-                struct keypos *kpos = hiarray_push(r->keys);
-                if (kpos == NULL)
+                if (!push_keypos(r, arg, arglen))
                     goto oom;
-                kpos->start = arg;
-                kpos->end = arg + arglen;
                 goto done;
             }
         }
@@ -353,11 +359,8 @@ void redis_parse_cmd(struct cmd *r) {
         goto error;
     }
 
-    struct keypos *kpos = hiarray_push(r->keys);
-    if (kpos == NULL)
+    if (!push_keypos(r, arg, arglen))
         goto oom;
-    kpos->start = arg;
-    kpos->end = arg + arglen;
 
     /* Special commands where we want all keys (not only the first key). */
     if (redis_argx(r) || redis_argkvx(r)) {
@@ -370,11 +373,8 @@ void redis_parse_cmd(struct cmd *r) {
                 goto error;
             if (redis_argkvx(r) && i % 2 == 0)
                 continue; /* not a key */
-            struct keypos *kpos = hiarray_push(r->keys);
-            if (kpos == NULL)
+            if (!push_keypos(r, arg, arglen))
                 goto oom;
-            kpos->start = arg;
-            kpos->end = arg + arglen;
         }
     }