gas: correct .irpc handling with empty string

Following 69cab370cf66 ("gas: adjust handling of quotes for .irpc") the
closing quote was mistakenly treated as the first quoted character.

gas/macro.c

diff --git a/gas/macro.c b/gas/macro.c
index a35e135..d0c90d3 100644
--- a/gas/macro.c
+++ b/gas/macro.c
@@ -1389,6 +1389,7 @@
 		      if (idx >= in->len)
 			break;
 		    }
+		  continue;
 		}
 	      sb_reset (&f.actual);
 	      sb_add_char (&f.actual, in->ptr[idx]);

gas/testsuite/gas/macros/irpc-quote.s

diff --git a/gas/testsuite/gas/macros/irpc-quote.s b/gas/testsuite/gas/macros/irpc-quote.s
index 0499f69..0326196 100644
--- a/gas/testsuite/gas/macros/irpc-quote.s
+++ b/gas/testsuite/gas/macros/irpc-quote.s
@@ -1,6 +1,6 @@
-	.irpc c, " ab" cd " ef"
+	.irpc c, " ab" cd " ef" ""
 	.print ">\c<"
 	.endr
-	.irpc c, "12 " 34 "56 "
+	.irpc c, "" "12 " 34 "56 "
 	.print ">\c<"
 	.endr