check_snmp: Fix regression introduced in #1867716

Bug #1867716 fixed what it meant to fix: broken perfdata strings.
Unfortunately some users relied on half-broken perfdata string where at
least the first token was OK. This patch do a two-way conversion
(string to double then back to string) instead and use the conversion
result for the performance data.

A possible caveat is that the string may change where it normally
shouldn't but the result should be somewhat similar.

NEWS

@@ -11,6 +11,7 @@ This file documents the major additions and syntax changes between releases.
 	Try to detect arguments passed via --with-ping[6]-command and set options accordingly (#2908236)
 	Fix memory leak in check_http for large pages (Jimmy Bergman - #2957455)
 	Fix compilation with GCC 2.96 (Konstantin Khomoutov - #2977105)
+	Fix regression introduced in #1867716 where partially valid performance strings would not be printed anymore
 	WARNINGS
 	Updated developer documentation to say that performance labels should not have an equals sign or
 	single quote in the label

plugins/check_snmp.c

@@ -122,6 +122,7 @@ main (int argc, char **argv)
 	int result = STATE_UNKNOWN;
 	int return_code = 0;
 	int external_error = 0;
+	double perftmp;
 	char **command_line = NULL;
 	char *cl_hidden_auth = NULL;
 	char *oidname = NULL;
@@ -351,10 +352,17 @@ main (int argc, char **argv)
 		if (nunits > (size_t)0 && (size_t)i < nunits && unitv[i] != NULL)
 			asprintf (&outbuff, "%s %s", outbuff, unitv[i]);
 
-		if (is_numeric(show)) {
+		/* Try a two-way conversion of show and add perfdata only if we get
+		 * something back at the end */
+		ptr = NULL;
+		perftmp = strtod(show, &ptr);
+		if (ptr != show) {
+			ptr = NULL;
+			asprintf(&ptr, "%0.9g", perftmp);
 			strncat(perfstr, oidname, sizeof(perfstr)-strlen(perfstr)-1);
 			strncat(perfstr, "=", sizeof(perfstr)-strlen(perfstr)-1);
-			strncat(perfstr, show, sizeof(perfstr)-strlen(perfstr)-1);
+			strncat(perfstr, ptr, sizeof(perfstr)-strlen(perfstr)-1);
+			free(ptr);
 
 			if (type)
 				strncat(perfstr, type, sizeof(perfstr)-strlen(perfstr)-1);