(cp 15d8a19) fixes #988 no more warning on PHP 7.3

This fix was initially only for branch 2.9, and branch master should have
received a new version of jshrink, but I forgot to do it before 2.10 and now
we have a regression in 2.10. This commit will be cherry-picked to 2.10 and
we also have it on master so that we don't have a new regression in 2.11 (in
case jshrink is not properly updated)

include/jshrink.class.php

@@ -192,7 +192,7 @@ class JShrink_Minifier
 				case "\n":
 					// if the next line is something that can't stand alone
                     // preserve the newline
-					if(strpos('(-+{[@', $this->b) !== false)
+					if($this->b !== false && strpos('(-+{[@', $this->b) !== false)
 					{
 						echo $this->a;
 						$this->saveString();
@@ -239,7 +239,7 @@ class JShrink_Minifier
 							if($this->a == '/' && ($this->b == '\'' || $this->b == '"'))
 							{
 								$this->saveRegex();
-								continue;
+								continue 3;
 							}
 
 							echo $this->a;