Add experimental semgrep rule 'if-incorrect-nil-err-return'

Put in experimental directory because of huge count of false positives.

.semgrep/experimental/if-incorrect-nil-err-return.yaml

@@ -0,0 +1,47 @@
+rules:
+  - id: if-incorrect-nil-err-return
+    languages: [go]
+    severity: WARNING
+    message: |
+      WARNING: A local variable '$ERR' is checked for nil, but a different variable is returned.
+      Ensure that the returned variable is the one that was checked or properly wrapped!
+    patterns:
+      - metavariable-regex:
+          metavariable: $ERR
+          regex: .*(?i)err # using .* to allow prefixes, because regex matching is left anchored.
+
+      - pattern: |
+          if $ERR != nil {
+              ...
+              return ..., $OTHERERR
+          }
+
+      - pattern-not: |
+          if $ERR != nil {
+              ...
+              return ..., $ERR
+          }
+      - pattern-not: |
+          if $ERR != nil {
+              ...
+              return ..., $ANYFUNC(..., $ERR, ...)
+          }
+      - pattern-not: |
+          if $ERR != nil {
+              ...
+              return ..., $ANYFUNC(..., $ANYFUNC1(..., $ERR, ...), ...)
+          }
+      - pattern-not: |
+          if $ERR != nil {
+              ...
+              $NEWERR := $ANYFUNC(..., $ERR, ...)
+              ...
+              return nil, $NEWERR
+          }
+      - pattern-not: |
+          if $ERR != nil {
+              ...
+              $NEWERR := $ANYFUNC(..., $ERR, ...)
+              ...
+              return ..., $ANYFUNC1(..., $NEWERR, ...)
+          }