Fix incomplete revert of broken IMF quoted string parsing (#28)

Cherry-pick fix from upstream libetpan PR #411.

A previous incomplete revert left broken error handling patterns in
mailimf_quoted_string_parse and mailimf_fws_quoted_string_parse.

The pattern `else if (r == MAILIMF_ERROR_PARSE) break; else { error }`
incorrectly breaks out of the parsing loop on parse errors. The correct
pattern is `else if (r != MAILIMF_ERROR_PARSE) { error }` which only
handles actual errors while allowing MAILIMF_ERROR_PARSE to continue
the loop naturally.

This restores the original working behavior of IMF field parsing.

Upstream: dinhvh/libetpan#411
Related: dinhvh/libetpan#406

Co-authored-by: Claude <noreply@anthropic.com>

Author: bengotow

Vendor/libetpan/src/low-level/imf/mailimf.c

@@ -1441,9 +1441,7 @@ int mailimf_quoted_string_parse(const char * message, size_t length,
         goto free_gstr;
       }
     }
-    else if (r == MAILIMF_ERROR_PARSE)
-      break;
-    else {
+    else if (r != MAILIMF_ERROR_PARSE) {
       res = r;
       goto free_gstr;
     }
@@ -1538,15 +1536,13 @@ int mailimf_fws_quoted_string_parse(const char * message, size_t length,
         goto free_gstr;
       }
     }
-    else if (r == MAILIMF_ERROR_PARSE)
-      break;
-    else {
+    else if (r != MAILIMF_ERROR_PARSE) {
       res = r;
       goto free_gstr;
     }
-    }
+  }
 
-    r = mailimf_dquote_parse(message, length, &cur_token);
+  r = mailimf_dquote_parse(message, length, &cur_token);
     if (r != MAILIMF_NO_ERROR) {
       res = r;
       goto free_gstr;