fix check_snmp regex matches

the multiplier function always tried to extract a number, even if the result
is a string because of using a mib.

before:
```
./check_snmp -H hostname -P2c -c public -o IF-MIB::ifAdminStatus.11466 -vvv -r 0
/usr/bin/snmpget -Le -t 10 -r 5 -m ALL -v 2c [context] [authpriv] 10.0.13.11:161 IF-MIB::ifAdminStatus.11466
IF-MIB::ifAdminStatus.11466 = INTEGER: up(1)
Processing oid 1 (line 1)
  oidname: IF-MIB::ifAdminStatus.11466
  response:  = INTEGER: up(1)
SNMP OK - 0 | IF-MIB::ifAdminStatus.11466=0;;
```

the regexp 0 matches, even if the actual result is "up(1)".

after this patch:
```
./check_snmp -H hostname -P2c -c public -o IF-MIB::ifAdminStatus.11466 -vvv -r 0
/usr/bin/snmpget -Le -t 10 -r 5 -m ALL -v 2c [context] [authpriv] 10.0.13.11:161 IF-MIB::ifAdminStatus.11466
IF-MIB::ifAdminStatus.11466 = INTEGER: up(1)
Processing oid 1 (line 1)
  oidname: IF-MIB::ifAdminStatus.11466
  response:  = INTEGER: up(1)
SNMP CRITICAL - *up(1)* |
```

1 files changed, 21 insertions, 2 deletions

diff --git a/plugins/check_snmp.c b/plugins/check_snmp.c
index 56bad88..d3968a2 100644
--- a/plugins/check_snmp.c
+++ b/plugins/check_snmp.c
@@ -1165,17 +1165,36 @@ nextarg (char *str)
 char *
 multiply (char *str)
 {
-        double val = strtod (str, NULL);
-        val *= multiplier;
+        char *endptr;
+        double val;
         char *conv = "%f";
+
+        if(verbose>2)
+                printf("    multiply input: %s\n", str);
+
+        val = strtod (str, &endptr);
+        if ((val == 0.0) && (endptr == str)) {
+                if(multiplier != 1) {
+                        die(STATE_UNKNOWN, _("multiplier set (%.1f), but input is not a number: %s"), multiplier, str);
+                }
+                return str;
+        }
+
+        if(verbose>2)
+                printf("    multiply extracted double: %f\n", val);
+        val *= multiplier;
         if (fmtstr != "") {
                 conv = fmtstr;
         }
         if (val == (int)val) {
                 sprintf(str, "%.0f", val);
         } else {
+                if(verbose>2)
+                        printf("    multiply using format: %s\n", conv);
                 sprintf(str, conv, val);
         }
+        if(verbose>2)
+                printf("    multiply result: %s\n", str);
         return str;
 }